Call transfer is not working?

Support Help
1

Tool Invocation: transfer_call

tool_call_id: transfer_call_ef54c5

{
  "number": "+192########",
  "option": {
    "type": "cold_transfer",
    "coldTransferMode": "sip_invite",
    "showTransfereeAsCaller": false
  }
}

tool_call_id: transfer_call_ef54c5

{“status”:“Transfer destination is not supported.. Please inform the customer that the transfer did not go through and offer to try again or assist them directly.”}

2

Hello @aamran The error “Transfer destination is not supported” means the phone number provided isn’t a valid transfer target. Common causes and fixes:

  1. Number format: Ensure the number is in E.164 format (e.g., +19201234567) or a valid SIP URI (sip:user@domain).

  2. SIP INVITE + custom telephony: If using custom telephony, non-E.164 numbers require enabling raw format in the transfer settings.

  3. Caller ID override: If showTransfereeAsCaller is enabled with sip_invite, your telephony provider must support caller ID override — otherwise the transfer fails.

    If this is not issue can you share Call ID.

Thank You

3

Looking at your transfer attempt, the error “Transfer destination is not supported” typically means the destination number format isn’t compatible with your SIP setup.

Here’s what to check:

  1. Verify the phone number format. Make sure it’s a valid E.164 format (like +1234567890) and doesn’t have extra characters or spaces.

  2. Check if the destination is actually reachable via SIP. Some numbers—like extensions, internal IDs, or certain VoIP endpoints—won’t work with cold transfers using sip_invite mode.

  3. Try switching to warm transfer instead. Change coldTransferMode to “bridge_transfer” to test if that destination works. This gives you more control before handing off the call.

  4. Confirm the number isn’t blocked or restricted in your carrier settings or Retell account permissions.

  5. If you’re transferring to an extension or internal number, you may need to use a different transfer type or provide the full SIP URI instead of just a phone number.

If the number works fine elsewhere, reach out to Retell support with your transfer_call_id (transfer_call_ef54c5) and they can check your account’s carrier compatibility. They might also need to whitelist certain destinations if you’re on a restricted plan.

4

Hello @Shah-Fazal The phone number is in E.164 format.

It was working fine previously, but now it is not working.

I have tried different setting but they are all failing.

call id = call_9169f2e872ee1ca22a1db402a6e

5

Hey @aamran Checking with the team.

6

@Shah-Fazal Any update on this ?

7

I identified the problem, outgoing calls were blocked from phone number config, which is why it wasn’t working.

8

Hello @aamran The transfer was rejected by a per-number country restriction on the originating DID, not by the transfer mode, the cold versus warm setting, or the Show Transferee As Caller toggle. That is why every variation you tried failed the same way, because they all run through the same pre-flight validation step before any SIP INVITE is generated.
What we observed on call_9169f2e872ee1ca22a1db402a6e:

  • The transfer_call tool returned the rejection in about 7 milliseconds, which is far too fast for any SIP signaling to have been attempted, and matches a synchronous validator path.
  • The originating DID is +17409134263 (US, area code 740, retell-twilio).
  • The transfer destination resolved to +19293903249 (US, area code 929 in New York City) via the predefined Transfer Call node in your flow.
  • At the time of the call, the DID had its Allowed Outbound Countries restricted to Pakistan only, so any non Pakistan destination was rejected up front and surfaced to the LLM as Transfer destination is not supported.

We can also see that the phone number record was edited shortly after this ticket was opened, and the Allowed Outbound Countries restriction is no longer present on the DID as of right now. So if someone on your side already cleared it, transfers to US destinations should succeed on the next attempt.

To confirm and prevent a repeat:

  • Open the dashboard, then go to Phone Numbers, then select +17409134263.
  • Under the number settings, check the Allowed Outbound Countries field. Either leave it empty so all supported destinations are allowed, or include every country you actually need to transfer to, for example United States and Pakistan.
  • Save, then place one test inbound and trigger the transfer path again.

A few related notes while you are in there:

  • The Allowed Outbound Countries field lives on the phone number itself, not on the agent, so switching agents on the same DID would not have helped.
  • The same validator covers a few other rejection causes that share the same error text, including invalid E.164 formatting and destination countries our retell-twilio plan does not cover. Your destination is a valid US number, so neither applies here, but it is worth knowing if you ever see the same string on a different route.

Thank You