# Telnyx Calling: UAC Connections — Full Documentation
> Complete page content for UAC Connections (Calling section) of the Telnyx developer docs (https://developers.telnyx.com).
> Root index: https://developers.telnyx.com/llms.txt · Lightweight index for this subsection: https://telnyx-openapi-ng.s3.us-east-1.amazonaws.com/llms/calling/uac-connections.txt

## UAC Connections

### UAC Connections

> Source: https://developers.telnyx.com/docs/voice/uac-connections.md

**SIP Attach** is the Telnyx product for connecting your existing PBX or SIP platform directly to Telnyx. To use SIP Attach, create a **UAC Connection**.

A UAC Connection lets Telnyx register to your PBX as a SIP endpoint. This is the reverse of a typical SIP Credential Connection, where your PBX registers to Telnyx.

After registration succeeds, calls can move between your PBX and Telnyx resources, such as AI Assistants or Call Control Applications, over SIP. This lets you connect existing SIP infrastructure to Telnyx services without adding a PSTN phone-number hop.

## Create a Connection

[POST /v2/uac_connections](/api-reference/uac-connections/create-a-uac-connection)

```bash
curl -X POST https://api.telnyx.com/v2/uac_connections \
  -H "Authorization: Bearer $TELNYX_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "connection_name": "My UAC Connection"
  }'
```

`connection_name` is the only required field. You can also provide `user_name` and `password` at creation time — if omitted, Telnyx auto-generates them.

```bash
curl -X POST https://api.telnyx.com/v2/uac_connections \
  -H "Authorization: Bearer $TELNYX_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "connection_name": "My UAC Connection",
    "user_name": "my-sip-user",
    "password": "my-sip-password"
  }'
```

## Connection Fields

> **Note:** `fqdn` and `registration_status` are automatically generated by Telnyx. `user_name` and `password` are auto-generated if not provided.

## External Settings (`external_uac_settings`)

These are the SIP credentials **Telnyx uses to register to your PBX**. The `proxy` field is required — it's the SIP server address Telnyx will register to. `username` and `password` are the SIP credentials for that registration. `transport` specifies the transport protocol (`TCP`, `UDP`, or `TLS`).

[PATCH /v2/uac_connections/\{id\}](/api-reference/uac-connections/update-a-uac-connection)

```bash
curl -X PATCH https://api.telnyx.com/v2/uac_connections/{id} \
  -H "Authorization: Bearer $TELNYX_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "external_uac_settings": {
      "proxy": "sip:pbx.example.com:5060",
      "username": "telnyx-reg",
      "password": "s3cret",
      "transport": "TCP"
    }
  }'
```

## Internal Settings (`internal_uac_settings`)

These route **inbound calls from the PBX to a Telnyx application**. The key field is `destination_uri` — a SIP URI that tells Telnyx where to send the call.

### `destination_uri` format

```
@.sip.telnyx.com
```

| Part | Description |
|------|-------------|
| `extension` | PBX extension or dial string |
| `applicationFQDN` | Telnyx application FQDN (e.g. `assistant-` for AI Assistant, `cc-app-` for Call Control, `stt-` for Speech-to-Text) |

**Examples:**

| destination_uri | Routes to |
|-----------------|-----------|
| `1006@assistant-abc123.sip.telnyx.com` | Extension 1006 → AI Assistant (UUID `abc123`) |
| `2000@cc-app-xyz789.sip.telnyx.com` | Extension 2000 → Call Control app (ID `xyz789`) |

[PATCH /v2/uac_connections/\{id\}](/api-reference/uac-connections/update-a-uac-connection)

```bash
curl -X PATCH https://api.telnyx.com/v2/uac_connections/{id} \
  -H "Authorization: Bearer $TELNYX_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "internal_uac_settings": {
      "destination_uri": "1006@assistant-abc123.sip.telnyx.com"
    }
  }'
```

## Full Example

Create a UAC connection with all settings in a single request:

```bash
curl -X POST https://api.telnyx.com/v2/uac_connections \
  -H "Authorization: Bearer $TELNYX_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "connection_name": "Office PBX",
    "user_name": "my-sip-user",
    "password": "my-sip-password",
    "external_uac_settings": {
      "proxy": "sip:pbx.example.com:5060",
      "username": "telnyx-reg",
      "password": "s3cret",
      "transport": "TCP"
    },
    "internal_uac_settings": {
      "destination_uri": "1006@assistant-abc123.sip.telnyx.com"
    }
  }'
```

## Manage Connections

```bash
# List all UAC connections
curl https://api.telnyx.com/v2/uac_connections \
  -H "Authorization: Bearer $TELNYX_API_KEY"

# Get a specific connection
curl https://api.telnyx.com/v2/uac_connections/{id} \
  -H "Authorization: Bearer $TELNYX_API_KEY"

# Delete a connection
curl -X DELETE https://api.telnyx.com/v2/uac_connections/{id} \
  -H "Authorization: Bearer $TELNYX_API_KEY"
```

---

## API Reference (UAC Connections)

### UAC Connections

- [List UAC connections](https://developers.telnyx.com/api-reference/uac-connections/list-uac-connections.md): Returns a list of your UAC connections. A UAC (User Agent Client) Connection registers Telnyx to your PBX — the opposite of a standard SIP trunk, where the PBX…
- [Create a UAC connection](https://developers.telnyx.com/api-reference/uac-connections/create-a-uac-connection.md): Creates a UAC connection. A UAC (User Agent Client) Connection registers Telnyx to your PBX — the opposite of a standard SIP trunk, where the PBX registers to…
- [Retrieve a UAC connection](https://developers.telnyx.com/api-reference/uac-connections/retrieve-a-uac-connection.md): Retrieves the details of an existing UAC connection.
- [Update a UAC connection](https://developers.telnyx.com/api-reference/uac-connections/update-a-uac-connection.md): Updates settings of an existing UAC connection.
- [Delete a UAC connection](https://developers.telnyx.com/api-reference/uac-connections/delete-a-uac-connection.md): Deletes an existing UAC connection.
- [Check a UAC Connection Registration Status](https://developers.telnyx.com/api-reference/uac-connections/check-a-uac-connection-registration-status.md): Checks the registration status for a UAC connection (`registration_status`) as well as the timestamp for the last SIP registration event (`registration_status_…
- [SIP Registration Status](https://developers.telnyx.com/api-reference/uac-connections/sip-registration-status.md): Returns the live SIP registration state of a UAC connection: whether it is currently registered, when it last registered, and the last response Telnyx received…