# Restrictions for Audio RTP Passthrough

Audio RTP Passthrough does not apply to every call, and turning it on changes how a few extension features behave. RTP (Real-time Transport Protocol) is the stream that carries the live audio of a call, so this feature is really about the path that audio takes between phones. Review the cases below before you rely on it in production.

## How Audio RTP Passthrough works

When you enable Audio RTP Passthrough on an Onsite Proxy, phones that register to Cloud Voice through that same proxy send audio to each other directly, peer-to-peer. The media skips both the PBX (Private Branch Exchange, the phone system that normally handles your calls) and the Onsite Proxy, which keeps that audio traffic off both components. That direct path is not always possible, though, and enabling it places some restrictions on the affected extensions.

:::note
Direct peer-to-peer audio only happens between two phones that are both registered to Cloud Voice through the same Onsite Proxy. That shared proxy is what makes the shortcut possible.
:::

To turn the feature on, see [Edit an Onsite Proxy Instance](/pbx/administrator-guide/edit-an-onsite-proxy-instance/).

## Calls that fall back to the PBX

In the following calls, audio is still relayed by the PBX even when Audio RTP Passthrough is enabled on the Onsite Proxy. The direct peer-to-peer path simply does not take effect, and no action is needed on your part: the call still connects normally through the PBX.

- Conference calls
- Paging group calls
- Video calls
- Calls set up through the attended transfer feature code
- Calls that are being recorded
- Calls being monitored by a third party
- Calls where the two parties do not share a common codec (a codec is the audio format both phones must agree on)

## Changes to extension behavior

For any extension that registers through an Onsite Proxy with Audio RTP Passthrough enabled, two areas of behavior change. Each is described below with an example and how to work around it.

### DTMF mode

DTMF (Dual-Tone Multi-Frequency) is the set of tones a phone sends when a caller presses keypad digits, and features such as remote pickup and transfers rely on it. The extension's DTMF mode is switched automatically to **Info** while Audio RTP Passthrough is enabled. If you later disable the feature on the Onsite Proxy, the mode reverts automatically to **RFC4733 (RFC2833)**.

:::caution
If you manually set the DTMF mode to anything other than **Info** while Audio RTP Passthrough is enabled, the passthrough will not work for that extension.
:::

### Feature codes

A feature code is a short dial string (for example `*4`) that triggers a built-in action such as call pickup or transfer. Feature-code dialing works differently for extensions behind Audio RTP Passthrough:

- **A single feature code on its own does not trigger its action.** To run the feature, the user appends `#` to the code.

  For example, `*4` normally lets a user pick up a colleague's ringing call. With Audio RTP Passthrough enabled, the user dials `*4#` instead.

- **A blind or attended transfer by feature code must be dialed as one continuous string**: the feature code and the destination number together, with no pause in between.

  For example, a blind transfer normally means dialing `*03`, waiting for the tone, and then entering the destination (such as `1000`). With Audio RTP Passthrough enabled, the user dials `*031000` all at once.

:::tip
Tell your extension users about these two dialing changes before you enable the feature. Otherwise their usual feature-code habits (a bare `*4`, or dialing the transfer code and then waiting for a tone) will appear to stop working.
:::