ISDN: What Does It Mean?

A digital phone-line standard from the pre-internet era, and why Fax.Plus never needs one.

Von

Sarah Mitchell

Veröffentlicht am

September 9, 2026

Zuletzt aktualisiert am

September 11, 2026

ISDN (Integrated Services Digital Network) is a digital phone-line standard that can carry voice, data, and fax over the same digital circuit.
A standard analog Group 3 fax machine generally can't connect to ISDN directly without a terminal adapter, since ISDN transmits digitally rather than as analog tones.
Fax.Plus needs neither an ISDN line nor a terminal adapter, since it sends and receives faxes entirely over the internet.

What Is ISDN?

ISDN (Integrated Services Digital Network) is a digital telephone-line standard that can carry voice, data, and fax over the same digital circuit, developed as a step up from purely analog lines. It was designed to move multiple types of communication over one connection at higher quality than a standard analog line could support.

Can a Fax Machine Use ISDN?

A standard analog Group 3 fax machine generally can't connect to an ISDN line directly, since ISDN transmits digitally rather than as the analog tones a fax machine expects over a POTS line or a landline. Getting a traditional machine working on ISDN typically requires a terminal adapter to bridge that gap.

ISDN vs. the Wider Phone Network

ISDN was one of several ways to connect into the PSTN, the broader circuit-switched network that also carries POTS and landline calls. It never became as universal as plain analog service, and most offices that once used it have since moved to internet-based connections instead.

Why Fax.Plus Skips This Entirely

Fax.Plus needs neither an ISDN line, a terminal adapter, nor any other physical telephony hardware, since it sends and receives faxes entirely over an internet connection. For the broader shift this reflects, see moving off legacy analog phone infrastructure.

A Common Case: An Old Office Phone System Still Running ISDN

An IT provider auditing a mid-size office's phone system finds a legacy ISDN line still active, originally installed to support a now-unused fax machine and a handful of old desk phones. Decommissioning it removes a recurring line charge the office had simply forgotten it was paying.

FAQs

Can a fax machine work over an ISDN line?

Only with help. A standard analog fax machine can't connect to ISDN directly because ISDN transmits digitally, so it typically needs a terminal adapter to bridge the two.