> ## Documentation Index
> Fetch the complete documentation index at: https://developers.telnyx.com/llms.txt
> Use this file to discover all available pages before exploring further.

# SIP Attach (UAC SIP Connections)

> SIP Attach connects your existing PBX or SIP platform to Telnyx. Create a UAC Connection so Telnyx registers to your PBX and routes calls to Telnyx applications.

**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 theme={null}
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 theme={null}
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 theme={null}
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

```
<extension>@<applicationFQDN>.sip.telnyx.com
```

| Part              | Description                                                                                                                       |
| ----------------- | --------------------------------------------------------------------------------------------------------------------------------- |
| `extension`       | PBX extension or dial string                                                                                                      |
| `applicationFQDN` | Telnyx application FQDN (e.g. `assistant-<uuid>` for AI Assistant, `cc-app-<id>` for Call Control, `stt-<id>` 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 theme={null}
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 theme={null}
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 theme={null}
# 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"
```