What Is FXS?

Q: Easy way to remember FXS

FXS = the side where a Subscriber (phone) plugs in, it supplies power to the analog phone.

Q: Can fax use an FXS port

Yes, with a gateway that supports T.38 fax protocol for reliable fax over VoIP.

Q: Does FXS support all phones

It supports analog phones, cordless phones, fax, and modems, but not ISDN phones or older proprietary digital phones.

2026-05-07 2 min readSIPPER Team

An FXS port on a gateway or PBX supplies -48V DC power, dial tone, and ring voltage to analog devices like a carrier line would. Used to connect old analog phones, faxes, and door phones.

Definition

FXS (Foreign Exchange Subscriber) means "the side where a subscriber (phone) connects." The gateway plays the role of the carrier office, supplying power to the subscriber endpoint.

Usage

Plug analog phones, fax machines, door phones, elevator phones, or paging systems into an FXS port. The gateway converts to SIP and sends to the IP PBX.

Related Concepts

Opposite of FXO (Foreign Exchange Office), which receives carrier trunks. Hybrids like Yeastar TG and Grandstream HT812 ship both FXS and FXO together.

Frequently Asked Questions

Easy way to remember FXS

Can fax use an FXS port

Does FXS support all phones

Ready to deploy?

SIPPER designs, deploys, and maintains end-to-end enterprise phone systems. Start with a free quote.

Request Quote View Solutions