Connecting your Telxi SIP Trunk to Retell AI

This guide shows how to connect a Telxi SIP trunk to Retell AI using Retell's recommended Elastic SIP Trunking method, so your voice agents can make and receive calls over the PSTN with your Telxi numbers. It covers both authentication methods supported by Telxi — Digest (username/password) and IP-based.

Architecture at a glance

Telxi connection details

Retell connection details

When using IP-based authentication, send these IP blocks to support@telxi.com for whitelisting.

Authentication methods

Prerequisites

• A Retell AI account (dashboard.retellai.com).
• An active Telxi SIP trunk with at least one DID, in E.164 format.
• Your SIP username/password (Digest), or Retell's IP blocks to whitelist (IP-based).

What you do on the Telxi side depends on the authentication method:

• Digest (recommended): Sign in at portal.telxi.com → SIP trunks, select your trunk, and copy the SIP username and SIP password. Nothing else is configured on the Telxi side — you'll enter these in Retell in Step 2.
• IP-based: Send the public signaling IP that Retell will use to support@telxi.com to be whitelisted. Telxi configures the whitelist for you; you then only need the SIP host (sip.telxi.com) and port (5060) when configuring Retell. Retell sends from published IP ranges (18.98.16.120/30, 143.223.88.0/21, 161.115.160.0/19) — provide the relevant address(es) to support.

For inbound calls, give Telxi support the destination to route your DID to — Retell's SIP server sip:sip.retellai.com (append ;transport=tcp if inbound doesn't connect). Telxi sets up the routing on their side.

1. In the Retell dashboard, open Phone Numbers.
2. Click + and choose Connect to your number via SIP trunking.
3. Enter:
   • Phone number — your Telxi DID in E.164 format (e.g. +1XXXXXXXXXX).
   • Termination URI — your Telxi SIP server, sip.telxi.com (port 5060).
   • SIP username and SIP password — from portal.telxi.com → SIP trunks (Digest). Leave blank if using IP-based auth with Retell's IPs whitelisted by Telxi.
4. Save. The number now appears in Retell and can be assigned to an agent.

1. Assign the imported number to the agent that should handle its calls.
2. Inbound test: call your Telxi DID from any external line; the call should reach your Retell agent. If it doesn't connect, check the origination routing with Telxi and append ;transport=tcp to the Retell SIP URI.
3. Outbound test: trigger an outbound call from the agent; confirm it routes through Telxi. If it fails, re-check the termination URI and credentials/whitelist.

Securing the connection (optional)

• Use TLS 1.2+ for SIP signaling: sip:sip.retellai.com;transport=tls. SRTP media encryption requires TLS transport.
• To validate Retell's server certificate over TLS, add Amazon's root CA to your trust store: G2-RootCA1.pem.
• mTLS (mutual TLS) is supported for enterprise setups; contact Retell support to enable it for your account.

Call transfer (SIP REFER)

Retell's transfer feature works over SIP trunking. For a cold transfer that shows the transferee's number as caller ID (SIP REFER), enable SIP REFER and PSTN transfer on your Telxi trunk.

Alternative: Dial to SIP URI

If elastic SIP trunking doesn't fit your setup, Retell also supports Dial to SIP URI: your system calls Retell's Register Phone Call API, receives a call_id, and dials sip:{call_id}@sip.retellai.com within 5 minutes. Note that Retell's built-in transfer feature is not available with this method (you implement transfers yourself). Most Telxi customers should use elastic SIP trunking above.

Troubleshooting

Reference

• Retell custom telephony: docs.retellai.com/deploy/custom-telephony
• Retell Twilio / Telnyx / Vonage SIP guides (use as references): docs.retellai.com/deploy/twilio
• Telxi portal: portal.telxi.com