(URGENT) - Call transfer tool executes but no outbound call is placed (PSTN never rings)

Support Help
1

Hey everyone — looking to see if anyone else has run into this with Retell.

We’re seeing an issue where the call transfer tool is invoked correctly, but no outbound call is ever placed.

What we’re observing:

  • Real PSTN phone calls (not web calls)

  • Transfer tool executes, but the destination never rings

  • Happens even when transferring to a direct personal mobile number (E.164)

  • Tested:

    • Cold and Warm transfers

    • SIP INVITE (not REFER)

    • Caller ID set to Retell Agent’s Number

    • Minimal prompt that explicitly forces the transfer tool

  • Same downstream telephony + Ringba setup works correctly with a VAPI-based agent, which suggests the destination and carrier are fine

Session ID from our own failed test:
call_9a80267f457977c843a910ffe70

Additional context from our internal testing: when using a Telnyx number and attempting a transfer, Retell returned a message indicating the transfer could not be completed due to permission issues. That lines up closely with what we’re seeing across tests.

At this point it feels like the transfer is being blocked at the telephony / permissions layer, rather than being a prompt, formatting, or destination issue.

Has anyone else:

  • Seen transfers fail even to direct PSTN numbers?

  • Needed account-level enablement for outbound transfer calls?

  • Run into provider-specific limitations with Retell transfers?

Appreciate any insight — happy to share more details if useful.

2

Based on the sources, transfer failures can occur due to several telephony-layer issues that match what you’re describing.

Permission and Provider Configuration

When using custom telephony (which includes Telnyx), the sources indicate that transfer failures can result from telephony provider permission issues(1). The disconnection reason telephony_provider_permission_denied specifically represents cases where SIP trunk authentication failed(1).

For transfers to work properly with custom telephony providers, you need to verify your configuration is correct(1). If the configuration is incorrect, you should delete the imported number and re-import it(1). If that doesn’t resolve it, you’ll need to check with your telephony provider to see what’s causing the error on their side(1).

Caller ID and Transfer Type Considerations

The sources note that for warm transfers, caller ID override uses SIP DIAL and sets from and P-Asserted-Identity headers to the user’s number(2). However, the telephony provider must support caller id override for this feature to work(2).

For cold transfers, it uses SIP REFER, and it’s up to the telephony provider to support caller id override for SIP REFER(2). If caller id override is not supported, the transfer would fail(2).

Specifically regarding Telnyx: Retell Telnyx numbers only support caller ID override when using SIP REFER via cold transfer(2).

Troubleshooting Steps

To diagnose the issue:

  1. Check your call history and detailed log for the disconnection reason and SIP error code(1)
  2. Verify your custom telephony configuration matches the imported information correctly(1)
  3. Confirm with Telnyx whether your account has the necessary permissions for outbound transfers
  4. Review whether your transfer type (warm vs cold) is compatible with your provider’s capabilities(2)

The permission error message you received from Retell when using a Telnyx number strongly suggests this is a provider-level configuration or permission issue rather than a Retell platform bug.

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