Twilio SIP transfer reports success but never dials outbound PSTN

Support Help
1

Hi Retell team,

I’m using Retell with Twilio Elastic SIP Trunking.

My agent successfully executes the transfer_call tool. The call log shows “Transfer connected” and the result is {“status”:“transferred successfully”}.

The transfer target is +1 (225) 315-6068.

The caller hears the transfer beep, but my phone never rings.

We have verified:

  • Twilio Elastic SIP Trunk is configured.
  • The phone number is attached to the trunk.
  • The Termination SIP URI is configured and active.
  • Retell credentials are attached.
  • Twilio Function is deployed.

Twilio never creates an outbound PSTN call. The Twilio logs only show the inbound call from Retell.

Could you please inspect our call logs and determine why the outbound PSTN leg is not being initiated?

2

Hey @stanfordandsonsacele can you send the relevant Call ID to check.

Thank You

3

The id call_2b5f270493df69101658cf2e65f

Stanford knockum
Owner

4

Hi Shah-Fazal,
The relevant Call ID is: call_2b5f270493df69101658cf2e65f
This is the call where the SIP transfer reported success, but the outbound PSTN number never rang.
Thank you.

Stanford knockum
Owner

5

@stanfordandsonsacele Checking with the team.

6

Hello @stanfordandsonsacele Two things going on here:

On the specific call you referenced (call_2b5f270493df69101658cf2e65f), no transfer was actually attempted — the caller hung up ~18s in during the agent’s greeting, before the LLM emitted transfer_call.
On adjacent calls from +12252552675 where transfer_call did fire, it’s returning failure (e.g., “Transfer call cannot be completed at this moment”). The cause is a typo in the outbound Termination URI stored on the Retell phone number. The correct Twilio Elastic SIP Trunk host format is <trunk-name>.pstn.twilio.com — your stored value has pst-n instead of pstn and twillo instead of twilio, so the outbound INVITE never resolves to Twilio and no child PSTN call is created.

To fix: in Twilio Console, copy the exact Termination SIP URI, then update the Termination URI on the +12252552675 phone number in Retell to match. Feel free to test afterward.

Thank You

7

How I get my new number to transfer calls to my personal number

8

I corrected the Termination URI to exactly match Twilio (stanfordsonsretell.pstn.twilio.com). Authentication is configured with the Retell credential list. Diana now attempts to transfer calls, but transfers to both another Retell agent (Leo) and my personal phone still fail. Can you review my account configuration or logs to determine why SIP INVITE transfers are not completing?

9

@stanfordandsonsacele Could you please share the Agent ID and new Call IDs? This will help us investigate the issue and review it with the team.

10

We already corrected the Outbound Termination URI to the proper Twilio format ending in pstn.twilio.com. Please verify the currently saved URI on the Retell phone number—not the historical value from the older failed calls—and run a new live transfer test. The transfer is still failing after the correction.

Stanford knockum
Owner

11

Hey @stanfordandsonsacele We can see that transfers are now correctly routing to stanfordsonsretell.pstn.twilio.com, but Twilio is rejecting our SIP INVITE with 403 Forbidden on both attempts today (calls call_3c10ca896d466751c3465bb5468 and call_4d62ff8b25ced5edc06c420f30b). No ring, no digest challenge, Twilio’s edge is refusing the call before it hits PSTN.
That 403 is from your Twilio Elastic SIP Trunk’s authorization, not Retell. In your Twilio Console → Elastic SIP Trunking → stanfordsonsretell trunk → Termination, please verify all three:

  1. Credential List — confirm a Credential List is attached to this trunk’s Termination (not just created on the account). It should contain a credential with username retell and the password you set on the Retell phone-number auth config. If you rotated the password on Retell’s side, make sure it matches Twilio’s Credential List.
  2. IP Access Control List — if you have an IP ACL attached, our outbound SIP egress must be in it. Retell publishes the current egress list at Connect Retell voice agents to custom telephony - Retell AI — please add all of those IPs.
  3. Termination status — the trunk shows “Enabled” and the “Termination SIP URI” matches exactly what’s on the Retell number.

Once Twilio accepts the INVITE, you’ll start seeing the outbound child call in your Twilio Voice logs. Reply to us with a fresh call_id after retesting and we’ll re-verify from our side.

Thank You