T.38: What Does It Mean?

The ITU-T standard that lets a fax transmission travel over VoIP in real time, and how it relates to Fax.Plus's own infrastructure.

Sarah Mitchell

Published on

September 9, 2026

Last updated on

September 11, 2026

T.38 is the ITU-T standard that allows a fax transmission to travel over a VoIP or IP network in real time instead of a standard analog phone line.
It works by converting a fax machine's tones into IP packets at one end and back into tones at the other.
Standard VoIP audio codecs are tuned for voice, not precise fax tones, which is why a protocol built specifically for fax over IP exists at all.

What Is T.38?

T.38 is the ITU-T recommendation that defines how a fax transmission travels over a VoIP or other IP-based network in real time, rather than a traditional analog phone line. It converts the precise tone sequence a fax machine sends into IP packets at the sending end, then reconstructs those tones at the receiving end before the fax data is decoded.

Why It Exists

A regular VoIP call compresses audio using codecs designed for the human voice, which tolerate small timing shifts and quality loss without a listener noticing. A fax connection can't tolerate that: its fax group and compatibility negotiation and page data rely on tone timing precise enough that standard voice compression often causes failed or garbled transmissions. T.38 exists specifically to avoid that problem by handling fax data as its own packet stream instead of treating it like ordinary voice audio, which is also how VoIP handles a fax call when it's set up correctly.

T.38 and Fax.Plus

Because Fax.Plus operates as a cloud fax service built on IP-based telephony rather than analog phone lines, its infrastructure deals with the same real-time, packet-based fax delivery problem T.38 was designed to solve. Fax.Plus does not publish which specific fax-over-IP signaling method it runs internally, so this should be read as context for how that class of connection works in general, not a claim about Fax.Plus's own implementation by name. For the internet fax category T.38 belongs to more broadly, see internet fax.

A Common Case: Keeping an Old Fax Machine Working on VoIP

A dental office replacing its office phone system with VoIP asks its IT provider whether the new setup supports T.38 before keeping its existing analog fax machine connected through an adapter, since a VoIP line without that support tends to produce failed or garbled faxes.

FAQs

Does T.38 improve fax reliability over VoIP?

Yes, generally. Standard VoIP audio codecs are tuned for voice and often distort the precise tones a fax needs, causing failed transmissions. T.38 converts those tones into IP packets instead, which handles the real-time timing a fax connection needs more reliably.

Is T.38 the same thing as internet fax?

Not exactly. Internet fax is the broader category of sending fax over an internet connection instead of an analog phone line. T.38 is one specific ITU-T standard within that category, built for real-time delivery over VoIP.

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