Single Prompt agent transfer_call not working

Support Help
1

Hi,

I’ve been watching tutorials and doing some reading but I don’t seem to figure out how to make the transfer_call work for me. I even asked AI as suggested but I still can’t figure it out.

Anyone with more knowledge could help me with this issue?

Thanks in advance!

2

Hey @VirtualAP you can review this guide ( Transfer call tool for single & multi-prompt agents - Retell AI ). Also, could you share more details about what specifically isn’t working? This will help us better understand the issue and assist you more effectively.

3

Hi and thank you,

The virtual agent makes the attempt but the transfer is not completed. The following message shows up in the history:

tool_call_id: call_yreYMqBVQnLAblTOxMrQbbYP

{“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.”}

4

Hey @VirtualAP The error “Transfer destination is not supported” means the transfer number format is invalid for your telephony setup. Check the following in your Transfer Call tool configuration:

  1. Number format: The destination must be in e.164 format (e.g., +12345678900) or a SIP URI (e.g., sip:user@retellai.com).
  2. Raw format: Only use “raw format” if you’re on custom telephony — it does not apply to Retell Telephony.
  3. Dynamic variables: If using a dynamic variable for the number, ensure it resolves to a valid e.164 or SIP URI at runtime.

If you have already checked this, please share the relevant Call ID and Agent ID so we can review the issue further.

Thank You

5

Call ID: call_01540c479e37ba602a224200ea1

Agent ID: agent_bb5b4f1a3b50a8372b44dd5368

6

@VirtualAP I have escalated this with the team for a review.

7

Hello @VirtualAP The transfer is being rejected before any dial attempt because the destination number on this transfer_call tool is +1 XXX XXX 6789, which is a Puerto Rico number. Outbound transfers to Puerto Rico aren’t currently supported from Retell-managed Twilio numbers (the inbound DID +1 XXX XXX 7857 is one of those), which is why you’re seeing the generic “Transfer destination is not supported” status come back in roughly 7 ms — it’s a pre-flight check, not a SIP/carrier failure.

Two options to unblock:

Thank You

8

Thank you for the explanation.

Could you please edit the destination number you wrote and only use the last 4 digits since it is a personal number?

My carrier does not provide or allow SIP trunk.

I guess I don’t have many options but I’m curious to know why Retell AI does not support Puerto Rico which uses the same country calling code (+1) as the United States (because it is a US territory) but it does support numbers in Asia, Oceania, and Europe.

Thank you!

9

Thank you for the editing!

I went ahead and changed the transfer destination to a number in the US but it did not go through. I think I’m missing a step.

It gave me the following message:

tool_call_id: call_xVzIHW55AbuDPhUV7A6z0VZO

{“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.”}

Call ID: call_34eb2a78efd4a6c3b40254d5c45

10

Just curious, are you doing a warm or cold transfer?

11

Hi. I am using a warm transfer

12

Are you using the agent phone number or the users number?

13

I am using the user’s number

14

Your SIP provider needs to support that. Test with the agents number and that will rule it out. I would guess it is that.

15

Thank you for the advice. I just tested it with the agent’s number but it did not work.

16

have you tested outbound calls to make sure that just even outbound calls work?

17

Hey @VirtualAP apologies for posting the full number,On the “why Puerto Rico?” question — The supported-destination list on the Retell-managed Twilio path is keyed by ISO country code. Even though Puerto Rico shares the +1 NANP prefix with the US, our number-parsing library resolves +1 787 / +1 939 to country PR, and PR isn’t in that list. The same applies to the other US territories (Guam, USVI, etc.) and most of the +1 Caribbean.

The list reflects the countries Twilio outbound was onboarded for on the managed path — not a deliberate exclusion of PR, just a country that hasn’t been added yet.

Since BYOC isn’t viable for you, and to head off the natural follow-up: the Retell-Telnyx managed path doesn’t help here either — its supported list is currently just US / IN / CA, so PR isn’t covered there either.

That leaves two immediate options:

  • Change the transfer destination to a number in one of the supported Twilio countries above, or
  • I can’t commit to a timeline, but I can let the team know of your request to support this destination.

Thank You

18

Thank you. I am not using outbound at this time

19

No worries about the full number issue @mark1 . You did not know. Thank you again for editing it.

  • I tested transferring to a supported Twilio country (USA) but it did not work either. I might have something incorrectly configured.
  • I would greatly appreciate it

Is it possible for you to help me figure out what aspect of the configuration I have wrong?

Thank you

20

Hey @VirtualAP can you share recent Call IDs?