# Fax Overview

Cloud Voice lets you attach a physical fax machine to your phone system so you can send and receive faxes just as you always have, while also giving you the option to receive incoming faxes as email attachments. This page explains how fax travels across a VoIP network, then walks through fax to email, fax detection, and the transmission settings you can adjust.

## T.38 virtual fax

Because Cloud Voice carries calls over the Internet rather than a traditional analog line, faxing relies on the T.38 protocol. T.38 packages a fax so it can be transmitted over a Voice over IP (VoIP) connection, an approach commonly called virtual fax or Fax over IP (FoIP).

A T.38 fax passes through three stages:

1. A fax machine sends a fax into a T.38-capable gateway, which acts as the sending (emitting) server.
2. The sending server breaks the fax down into an encoded image, streams that T.38 data across the Internet in real time, and delivers it to another T.38-capable server, for example a PBX (Private Branch Exchange, the phone system that routes your calls), that acts as the receiving server.
3. The receiving server converts the incoming T.38 stream back into an analog signal and passes it to the destination fax machine.

:::note
A traditional fax rides on a steady analog line, while a T.38 fax is broken into data packets and reassembled at the other end. That is why the Fax over IP settings further down this page exist: they let you tune the transmission so a real-time network hiccup does not corrupt the fax.
:::

The diagram below shows this flow end to end.

![Fax data moving from a sending fax machine and gateway across the Internet to a receiving PBX and fax machine using the T.38 protocol](/images/pbx/t.38-diagram.png)

## Fax to email

With a conventional setup, every fax lands as a printed page at a fax machine. Cloud Voice adds a fax to email option that delivers each incoming fax to a mailbox as a PDF instead.

Receiving faxes by email offers several advantages:

- Your faxes stay private, with no paper sitting in a shared output tray.
- You can read faxes the moment they arrive, from wherever you are.
- You avoid the cost of dedicated hardware, printer paper, routine maintenance, and a separate fax line.

:::tip
For most users, fax to email is the simplest choice: there is no fax machine to buy, no paper to reload, and no dedicated fax line to pay for. Faxes arrive as PDF attachments you can save, search, or forward like any other email.
:::

## Fax detection

Fax detection automatically determines whether an inbound call is a voice call or a fax. It is especially useful when voice and fax calls share the same line:

- When the system detects a fax signal, it routes the call straight to the fax destination you have configured.
- When no fax signal is present, the system treats the call as an ordinary voice call.

:::note
Turn on fax detection when a single number or line has to carry both voice calls and faxes, so you do not need a separate fax line. Choose a fax destination first, because that is where every detected fax is sent. If no destination is set, detected faxes have nowhere to go.
:::

## Fax over IP (FoIP) settings

The following settings let you fine-tune how faxes are transmitted across a VoIP network:

- **T.38 Support**: Turns the T.38 protocol on or off for an extension or trunk, depending on your needs.
- **T.38 Max BitRate**: Sets the maximum bit rate for fax transmission. The default is `14400`.
- **No T.38 Attributes in re-INVITE SDP**: Controls whether T.38 attributes are included in the re-INVITE packet's SDP. SDP (Session Description Protocol) is the part of a SIP (Session Initiation Protocol) message that describes the media in a call, and a re-INVITE is the message that renegotiates a call in progress, for example when it switches from voice to fax.
- **Error Correction Mode**: An optional transmission mode. Error Correction Mode (ECM) detects and repairs errors that occur during a fax, such as those introduced by noise on the telephone line.

:::caution
T.38 Support must be enabled on both the extension and the trunk that carry the fax, and the far end has to support T.38 as well. If either side has it switched off, the two systems cannot negotiate T.38 and the fax fails. Likewise, do not set T.38 Max BitRate higher than both the fax machine and the trunk can handle: a rate above what an endpoint supports causes faxes to fail. Leave it at the `14400` default unless your provider advises a different value.
:::

:::tip
Leave Error Correction Mode enabled unless you are troubleshooting. It catches and re-sends portions of a fax that were damaged by line noise, which reduces garbled or incomplete pages.
:::