A UAC (User Agent Client) Connection registers Telnyx as a SIP endpoint on your external PBX.
This gives your Telnyx resource, an AI Assistant, Call Control Application, or other telephony resource, native PBX presence.
It appears as an extension on your PBX, with no PSTN hop: lower latency and lower cost.
How UAC differs from FQDN connections
Before you begin, it helps to understand the directional difference:
FQDN / IP connections: Your PBX registers to Telnyx, or Telnyx accepts calls from your PBX based on IP Authentication. Telnyx is the server (UAS) and the PBX the client (UAC).
UAC connections: Telnyx registers to your PBX as an extension. Your PBX is the server (UAS) and Telnyx the client (UAC).
This affects how inbound calls are routed (covered in Step 2) and what you need to configure on the PBX side.
Before You Start
What you need from your PBX (customer side)
Before creating a UAC Connection, collect the following from your PBX administrator or PBX management console:
Item | Notes |
SIP username and password | Credentials for the extension Telnyx will register as |
SIP proxy address | Where Telnyx sends SIP REGISTER requests (e.g., pbx.example.com:5060) |
Transport protocol | UDP, TCP, or TLS — must match your PBX |
Auth username (if different) | Some PBX systems use a separate username for SIP digest auth |
Outbound proxy (if applicable) | If your network routes SIP through an intermediary |
From User (if applicable) | If your PBX requires a specific caller identity in the SIP From header |
What you need to configure on your PBX (customer side)
On your PBX, you must:
Create a SIP extension that Telnyx will register as (e.g., extension 1001)
Allowlist Telnyx signaling IPs — your PBX must accept SIP REGISTER and INVITE requests from Telnyx's IP ranges.
What you need on the Telnyx side
A Telnyx account
Telnyx resource (such as a Call Control Application, AI Assistant, or SIP-enabled application) that will handle calls on this connection.
Step 1 — Select Connection Type
Navigate to Voice → SIP Trunking → Create SIP Connection in the Mission Control Portal.
Select the UAC (User Agent Client) tile (marked BETA).
Enter a Name for your connection (e.g., PBX-uac).
Click Next.
Step 2 — Authentication and Routing
Step 2 has two sections: Internal UAC settings (Telnyx side) and External UAC settings (your PBX side).
Understanding the directional model
A UAC connection handles calls in two directions, and two different fields govern each direction:
Telnyx → PBX (outbound): uses the auto-generated SIP Subdomain
PBX → Telnyx (inbound): uses the Internal SIP URI you configure
Keep this in mind as you fill out the Internal UAC settings below, the two fields aren't redundant, they handle opposite directions of traffic.
Internal UAC settings (Telnyx side)
These fields define how Telnyx identifies this connection and how it handles inbound calls from the external peer.
Field | Required | Description |
SIP Subdomain | Read-only | Auto-generated by Telnyx ( |
Receive SIP Subdomain calls | Yes | Controls who can place calls to the subdomain. Defaults to From Anyone. When enabled, Telnyx accepts inbound SIP calls addressed to this subdomain and routes them to the Internal SIP URI below. |
Username | Yes | Internal SIP username (Telnyx-side identity). |
Password | Yes | Internal SIP password. |
Internal SIP URI | Yes | The Telnyx SIP URI that receives inbound calls from your PBX. Can be any valid SIP URI in your Telnyx account — typically the URI of the AI Assistant or Call Control Application that should answer these calls. See the portal tooltip for format requirements. |
Why the Internal SIP URI matters Because Telnyx registers to your PBX (rather than the other way around), inbound calls arrive via the SIP subdomain.
The Internal SIP URI tells Telnyx where to deliver them, typically your AI Assistant or Call Control Application. If this is wrong, registration will succeed but calls won't route.
External UAC settings (PBX side)
These fields configure how Telnyx registers with your external PBX.
Username | Yes | SIP username for authenticating with your PBX. |
Password | Yes | SIP password for digest authentication. |
Proxy | Yes | SIP proxy address of your PBX (e.g., pbx.example.com:5060). Where Telnyx sends SIP REGISTER requests. |
Auth Username | No | Separate username for digest auth, if your PBX uses one. Defaults to Username. |
From User | No | User portion of the SIP From header in outbound requests. Defaults to Username. |
Outbound proxy | No | SIP proxy for routing outbound requests through an intermediary, if required. |
Expiration (sec) | No | Registration refresh interval. Telnyx re-registers at this interval. |
Transport | No | UDP, TCP, or TLS. Must match your PBX configuration. |
Click Next.
Steps 3–6 — Configuration, Inbound, Outbound, Numbers
Configure these steps the same way you would for an FQDN connection:
Step 3 — Configuration: Codec preferences, SRTP, and other SIP settings.
Step 4 — Inbound: Inbound call routing and handling rules.
Step 5 — Outbound: Outbound call routing rules.
Step 6 — Numbers: Assign phone numbers to this connection. (Optional)
UAC inbound routing is different For IP-based and FQDN connections, inbound calls are routed based on source IP or resolved hostname.
For a UAC connection, Telnyx is a registered endpoint on the PBX, calls arrive via the SIP subdomain and are routed according to the Internal SIP URI you set in Step 2.
Make sure that URI points to the correct Telnyx resource.
To edit your configuration later, navigate to Voice → SIP Trunking, select your UAC connection, and go to the UAC Settings (Beta) tab.
Edit or Delete a UAC Connection
Edit credentials or proxy settings
Navigate to Voice Suite → SIP Trunking → SIP Connections in the Mission Control Portal.
Select your UAC connection.
Click Edit to update any field in Step 2 — for example, changing the Password after a PBX credential rotation, updating the Proxy address after a network change, or adjusting the Expiration (sec) value.
Changes to External UAC settings trigger a re-registration with your PBX.
Delete a connection
Navigate to Voice Suite → SIP Trunking → SIP Connections in the Mission Control Portal.
Select your UAC connection.
Click Delete and confirm.
Deleting a connection immediately deregisters from the PBX. Any phone numbers assigned to the connection will be unlinked — reassign them to another connection before deleting if you want to preserve inbound service.
Troubleshooting
Connection stuck in "Registering"
Verify your PBX is reachable at the Proxy address from the public internet.
Check that the port is correct (5060 for UDP/TCP, 5061 for TLS).
Confirm no firewall is blocking SIP traffic from Telnyx IPs.
Verify the Transport setting in Step 2 matches what your PBX expects (UDP, TCP, or TLS).
Registration succeeds but calls don't route
On the PBX side: Verify your PBX dial plan routes calls to the registered extension correctly. Confirm the extension is assigned to the right user or hunt group.
On the Telnyx Mission Control Portal: Confirm Receive SIP Subdomain calls is set correctly in Step 2 — this controls whether Telnyx accepts inbound calls addressed to the SIP subdomain. Verify your telephony resource (AI Assistant, Call Control Application) is correctly linked to this connection and the Internal SIP URI points to it.
Registration expires frequently
If your PBX rate-limits registrations or you see frequent re-registration cycles:
Edit the connection in the Mission Control Portal.
Increase the Expiration (sec) value in Step 2 (e.g., from 3600 to 7200).
Check your PBX's minimum and maximum registration interval settings — some PBX systems override the requested expiry.
Need Help?
If you're still having trouble, contact Telnyx support at support@telnyx.com or open a ticket in the Mission Control Portal. Include your Connection ID and any SIP logs from your PBX for faster resolution.