Best Fax APIs in 2026: Compare 10 Providers and Their Webhooks

Quick comparison

Fax.Plus leads for cloud-first teams; the others fit narrower integration or deployment needs. The right shortlist depends on how much of the workflow you need to control, how your app will recover from interruptions, and whether you want a cloud service or an API connected to fax infrastructure you operate.

What to look for in a fax API

An API can support fax sending and still be a poor fit for a production workflow. Use these questions to assess the complete integration:

1. Can it cover the whole fax lifecycle and team setup? Check for sending, receiving, status lookup, document retrieval, number purchase and assignment, and any member roles, quotas, or invitations your workflow needs. Also confirm which admin actions are available through the API and which still require the portal.
2. Can your team get started securely? Look for clear authentication instructions, usable examples, test access, realistic test data, and an implementation-support path. Confirm how API credentials are issued, scoped, and revoked, and whether that model fits your organization's security requirements.
3. What happens after a fax event? Check whether notifications cover sent, received, failed, and, if your workflow needs it, individual inbound pages.
4. Can you operate the webhook in production? Look for signed requests, retry behavior, event logs, replay, endpoint configuration, filtering, and guidance for duplicate events.
5. Does the deployment model fit? A hosted REST API, a managed enterprise service, and a locally operated fax server create different implementation and procurement work.

What makes a fax API production-ready?

A complete API covers more than sending

Sending a fax is one step. A two-way application may also need to receive inbound faxes, list them, download the documents, check transmission status, manage numbers, and stop or resend work. Before comparing endpoint counts, draw the workflow your application needs from start to finish.

Then check where the fax document itself travels. Some callbacks include useful metadata but leave the application to retrieve the PDF or TIFF in a separate API call. That separation can help with security and payload size, but your integration must store the event, fetch the document, and handle a temporary download failure.

A test environment should match the question you need to answer

“Sandbox” can mean very different things. It may return mock responses, accept test numbers, send a real fax, or let you test only account setup. Ask which parts of production it exercises, whether the test account has the same features and limits, and what data the provider retains.

Fax.Plus API sandbox access is available by request through the API contact form, and the team arranges it. For other providers, ask whether the sandbox sends real faxes or returns mock responses.

Keep fax retry and webhook retry separate

If a recipient's fax machine is busy, the fax provider may retry the transmission. If your server times out while receiving the status notification, the provider may retry the webhook. Those are two separate queues, with different failure reasons and recovery actions.

Fax.Plus also offers an Auto Resume on Partial Send setting for eligible multi-page transmissions. When enabled, the continuation can start at the next unsent page, so the recipient may receive the fax in more than one transmission. This is a fax-delivery behavior, separate from webhook retry or replay.

Your webhook receiver should return a successful HTTP response quickly, save or queue the event, and process it asynchronously. It should also tolerate duplicates. A retry may deliver the same event more than once, and replay resends an event that your system did not process. In software, handling the same event safely without duplicating work is called idempotency.

Fax API webhooks: the questions to ask

A webhook is an automatic HTTP request that one service sends to your application when an event happens. Instead of repeatedly asking, “Did that fax finish yet?”, your system receives a message when the provider has an update.

Which events can you receive?

At minimum, check whether the provider sends events for outbound completion and inbound faxes. Then look at the payload and the meaning of each status. A “fax sent” event may mean that the fax job has finished, not that it succeeded. Your application still needs to inspect the actual delivery status.

Some workflows benefit from page-level events. For example, a team can start reviewing an inbound referral packet before the sender finishes transmitting every page. Fax.Plus documents a page-received event when Fax Streaming is enabled. Documo's event reference also lists an inbound-page event. Confirm how the full document is retrieved and what happens if page events arrive before the completed-fax record.

How do you verify that a webhook is genuine?

HTTPS protects the connection while data travels between servers. A signature gives your receiver another check: it can verify that a request was signed with a secret shared by the provider, and that the request body was not changed. Fax.Plus documents HMAC-SHA256 signatures for its current webhook API, with an endpoint-specific secret and a timestamp check. eFax's older API portal guide describes optional HMAC authentication. Retarus supports authentication options on HTTP status push, including Basic, Digest, and OAuth 2.0.

For any provider, confirm which authentication option applies to the exact API version and product you are buying. Do not treat IP allowlisting or a hard-to-guess URL as a substitute for verifying signed messages where signing is available.

Can you see and recover a missed event?

Retries help with temporary outages, but they cannot answer every support question. A delivery log helps an engineer see whether the service sent an event, what response your endpoint returned, and whether the provider will try again. Replay lets an authorized user resend a missed event after fixing the receiver.

Fax.Plus provides webhook delivery logs, automated retries, endpoint failure alerts, and self-serve replay, including replaying events from a selected time range. Documo sends an email after its scheduled retries are exhausted. InterFAX's API references include a way to resend a callback. For other providers, ask to see the production log and replay workflow, and clarify whether retry is automatic, manual, or both.

Can you route events to the right customer or fax number?

If your application supports several customers, departments, or fax numbers, ask how webhook endpoints can be separated. A number-to-team setting in the fax portal is not necessarily the same as filtering API events by fax number.

Fax.Plus supports filters by event and number, and its portal can manage multiple endpoints, helping teams route events across fax numbers and workflows. Documo documents account-level and number-level webhook setup. WestFax documents line-specific callback configuration. Also confirm how new numbers inherit settings.

The 10 fax API providers compared

1. Fax.Plus

Fax.Plus is a cloud fax API for teams that want to send and receive faxes, manage corporate accounts, and handle event delivery from their applications. Signing is built into the same suite: any fax in the inbox or sent folder can be forwarded to Sign.Plus, Alohi's own e-signature product, to sign it yourself or request signatures from others under the same account, and the mobile apps let you sign a received fax and send it straight back.

The webhook experience includes event subscriptions, per-number routing, endpoint-specific HMAC signatures, delivery identifiers, an event catalog, delivery logs, automated retries, replay for failed events or a selected time range, and alerts when an endpoint starts failing. With number filters, each endpoint receives events only for the fax numbers it lists, so a platform serving several customers can give each customer's numbers their own endpoint and secret. Fax Streaming adds inbound page notifications. Teams can inspect delivery attempts in the portal and replay events once a destination is ready, which makes it easier to investigate a missed notification or route events across multiple fax numbers. Its current /v3/webhooks API sits alongside the legacy webhook API during the transition; see the webhook documentation for implementation details.

Alongside fax operations, the REST API handles account administration: corporate members, roles and fax quotas, invitations, fax-number purchase and assignment, and contacts and groups, all through permission-scoped endpoints on the Enterprise API. See the accounts, number management, and number purchase references.

Fax.Plus also publishes platform availability on its status page, where teams can check service updates.

For sending and scale, the API supports one or more destinations in a send request and configurable fax-transmission retry count and delay, as shown in the send-fax example. Fax.Plus also offers Auto Resume on Partial Send for eligible multi-page faxes when enabled. Fax.Plus publishes plan-level page, file, and concurrent-transmission limits in its pricing details, with Enterprise options for higher volume. Fax status records and confirmation reports help reconcile individual transmission outcomes.

Teams that do not need a custom backend integration can also use no-code app connections such as Zapier. That is a practical alternative for supported workflows.

Real-time fax streaming

With fax streaming enabled, Fax.Plus sends a fax_page_received event the moment each page of an inbound fax is received, and that page is available right away through the API as a single-page file. Intake systems can classify or route a referral packet from page 1 while the rest is still arriving, instead of waiting for the full document. Streaming is enabled on request for the whole account, and it works on HIPAA accounts, where the event carries fax details rather than document content.

Connect AI assistants through MCP

The Model Context Protocol (MCP) lets compatible AI assistants connect to external tools. Fax.Plus offers a hosted MCP server that lets an assistant send faxes, check incoming documents, track delivery, and manage contacts through natural-language requests. One endpoint works across supported MCP clients, so teams can make fax actions available inside AI workflows without maintaining a separate connector for every assistant. OAuth is available on paid Fax.Plus plans; Enterprise also supports Personal Access Tokens with configurable scopes. Use MCP for assistant-led fax tasks, and the REST API for application-controlled workflows such as webhook configuration, number provisioning, and corporate member administration. Learn more about the Fax.Plus MCP server and its developer documentation.

The REST API and webhooks are available on Enterprise, while MCP is available on paid plans starting with Basic. Learn more about Fax.Plus Enterprise and its HIPAA fax workflow.

Best for: Cloud-first teams that need REST API automation, recoverable webhook operations, and a direct way to connect fax actions to AI assistants.

2. eFax Enterprise

eFax Enterprise combines a cloud REST fax API with account and number provisioning. Its public API page describes OAuth 2.0, sending and receiving, fax-image and metadata retrieval, webhooks, and a private developer portal with a sandbox. Fax.Plus also exposes member and number administration through its REST API; eFax may suit organizations that already use eFax Corporate and prefer its private developer-portal model.

The API page describes secure inbound and outbound notifications. A portal quick-start guide also explains receiving faxes through a posted URL and says HMAC authentication can be required. Because that guide is older than the current marketing page, ask eFax which signing, sandbox, event, and delivery-log features are available in the API environment being quoted. The portal is private, so a developer may need an account or sales contact before seeing the full reference.

Best for: Larger organizations looking for managed cloud fax with API-based faxing, account administration, and number provisioning.

3. iFax

iFax publishes a REST API reference covering outbound faxing, received faxes, status checks, document downloads, and number operations. It uses an account API key and JSON responses. The documentation also describes webhook behavior: global URLs for sent and received faxes, postflight notifications after a fax completes, and up to five retries with a published backoff schedule.

The global event URLs suit a straightforward account-wide setup. Teams that need signed requests, replay, or delivery-history tools should ask iFax how those controls fit their account and integration.

Best for: Developers who want an API-key REST integration with account-wide webhook URLs.

4. Retarus

Retarus offers a regional REST API for application faxing, alongside other enterprise integration options. Its manual describes status reports that can be polled or pushed to a customer endpoint. You can set an HTTP status-push URL for an individual job or configure a default account URL. The documented authentication choices include Basic, Digest, and OAuth 2.0. Status reports remain available for retrieval for up to 30 days or until manually deleted.

This fits organizations that need regional endpoints, report retention, and authentication configured as part of a managed service. Retarus asks developers to contact the company for a test account. Its public manual does not describe webhook delivery replay or an event-by-event dashboard, so ask how endpoint failures are observed and recovered in the service you are evaluating.

Best for: Enterprise teams that want regional service options and configurable status delivery as part of a provider-led integration.

5. Concord

Concord's developer materials cover fax and document-exchange integrations, including APIs that accept incoming documents and return processed files through HTTP push or pull. Some developer resources require a portal login, and public material spans different documentation areas and API generations. Before comparing endpoint details, confirm which Concord product, API version, and deployment you would use.

One public outbound notification guide recommends implementing a pull-based status check as a backup to push. It notes that a push can be delayed by several minutes or delayed when the destination server is unavailable. Plan a reconciliation path rather than assuming every notification will arrive exactly when your app expects it. Ask Concord for current webhook authentication, retry, replay, and delivery-log details for the specific service under consideration.

Best for: Organizations evaluating Concord's broader document-exchange and enterprise integration capabilities, especially when fax is part of a larger program.

6. InterFAX

InterFAX publishes REST references for outbound and inbound fax operations. The documentation covers sending, retrieving status, working with inbound documents, and other job-management actions. Its API reference also includes a callback-resend operation, giving developers a documented route to request a callback again.

Before building around it, clarify which callback events it covers, whether failed deliveries are retried automatically, and how your team can inspect past attempts. Teams that require signed callbacks, filtering, and portal replay should confirm how those controls work in the InterFAX service they plan to use.

Best for: Developers who want a direct REST API for both inbound and outbound fax workflows and can confirm callback operations with the provider.

7. WestFax

WestFax's developer portal offers free sandbox credentials, mock REST APIs that follow production request paths and response shapes, and examples developers can copy. The sandbox lets a team simulate callback outcomes without sending a real fax.

The quick-start guide explains an outbound flow in two parts: WestFax calls a URL when the job finishes, then the application fetches detailed per-recipient status through another API call. Inbound callbacks similarly tell the app a fax arrived, after which it retrieves metadata and the PDF. The guide recommends returning a successful response quickly and processing work asynchronously; it also recommends idempotent handling because a slow endpoint may receive the callback again. Confirm production authentication, callback retry schedule, signing options, and event replay before launch.

Best for: Developers who want to build and exercise API logic in a mock environment before provisioning production access.

8. RightFax, now OpenText Fax

RightFax is now marketed by OpenText as OpenText Fax. OpenText describes on-premises, hybrid, and private-cloud deployment options. This makes it relevant for organizations that already operate fax infrastructure or need control over where the fax server runs, but it is a different comparison from a hosted, self-serve cloud API.

OpenText's version comparison chart documents Web API enhancements, including configurable retry count and retry interval properties. Earlier product documentation describes API operations for sending, receiving, checking status, routing, forwarding, retrying, and canceling faxes. Detailed API references are often tied to the server version and customer access, so confirm the supported version, licensing, hosting responsibility, and upgrade path with OpenText or its implementation partner.

Best for: Organizations that need an API connected to a customer-operated or hybrid fax-server environment.

9. Documo

Documo's webhook documentation describes configuration at either the account or fax-number level, a range of inbound and outbound fax events, and optional HTTP Basic authentication. It publishes a retry schedule of seven attempts over a longer backoff window. If all attempts fail, the documentation says an email is sent with error and webhook data. The event reference also identifies webhook-request completion and failure events.

The webhook is only part of the document flow. Inbound events can arrive page by page, while the application may retrieve the fax file through the API. Plan for separate event processing and document download, and confirm how permissions, retention, and retries behave for your account. Documo's public sources include both portal setup instructions and API references; make sure your team is reviewing the exact product and API configuration included in its quote.

Best for: Teams that want number-level webhook routing, with a separate API step for document retrieval.

10. SRFax

SRFax's developer guide describes an HTTPS POST API with JSON or XML responses. Its action-based operations cover queuing a fax, checking one or more fax statuses, retrieving inbox and outbox items, downloading fax files, and managing queued work. The provider also publishes downloadable client classes and examples.

For outbound updates, SRFax documents an sNotifyURL parameter that posts fax-status fields to a URL after a job is queued. The API documentation says developers can still poll for status if the notification is missed. This callback-plus-polling fallback works, but the public guide does not provide the same operational detail as a modern event portal. Ask about inbound notifications, callback authentication, delivery retries, and whether there is a log or manual replay option for your account.

Best for: Teams that are comfortable with an action-based API, downloadable code examples, and a polling fallback for outbound status notifications.

Cloud fax API, managed service, or fax server?

The API protocol is only part of the decision. The deployment model affects implementation, ownership, support, and migration.

• Cloud fax API: Your application calls a hosted service, and the provider operates fax delivery infrastructure. Fax.Plus, eFax, iFax, InterFAX, WestFax, Documo, and SRFax publish cloud API paths. Verify service regions, number coverage, throughput, retention, support, and account gates against your requirements.
• Enterprise managed service: A provider may help scope regional endpoints, account setup, routing, test access, and operational support. Retarus and Concord fit this model. Confirm which services are included in the contract and who owns the integration work.
• Customer-operated or hybrid fax server: The API connects to a server environment that your organization or implementation partner operates. RightFax/OpenText is relevant here. That model may offer more infrastructure control, but it also means planning for versioning, hosting, updates, monitoring, and support ownership.

These categories overlap. A provider may offer more than one delivery path, and a managed cloud service still needs technical onboarding. Ask the vendor to map the exact product and deployment in your quote to the API documentation you are reviewing.

A fax API evaluation checklist

Compare providers on the essentials:

• Workflow coverage: Can the API send and receive faxes, report status, return documents, and manage fax numbers and team access?
• Webhook operations: Are events clearly defined and signed, with retries, delivery logs, replay, routing filters, and safe duplicate handling?
• Fax outcomes: Can you track recipient-level results and retrieve confirmation reports? Keep fax transmission retries separate from webhook delivery retries.
• Developer access: Are documentation, SDKs, test access, authentication, and implementation support a good fit for your team?
• Security and deployment: Do access controls, data handling, retention, residency, regulatory terms, and cloud or hybrid options match your requirements?
• Scale and cost: Can you forecast page, file, concurrency, user, and number limits, and understand high-volume pricing?
• Automation fit: Does the provider connect with your existing tools, including no-code workflows or MCP for AI-assisted tasks?

Our verdict

For cloud-first teams that need an application API, operational webhooks, and a path into AI-agent workflows, Fax.Plus is our top recommendation. The REST API supports application-controlled fax operations. Signed, filterable webhooks give teams visibility into event delivery and recovery. The hosted MCP server connects compatible assistants to everyday fax tasks. Together, these capabilities give teams one platform for established backend integrations and newer conversational workflows.

Other providers fit narrower needs: eFax for organizations already on its Corporate service, and RightFax/OpenText for teams that need customer-operated or hybrid fax infrastructure. For teams building a modern cloud fax workflow across APIs, account and number administration, webhooks, and AI assistants, Fax.Plus is the strongest overall choice in this comparison. Use the Fax.Plus API for application integrations and MCP to bring fax into AI-assisted workflows.