Warm Transfer Caller ID shows Retell Agent's Number instead of User's Number on outbound calls (403 error)

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1

We’re running outbound calls via Retell using a an external Twilio number (+16505296559). When the AI agent qualifies a lead and initiates a warm transfer to our Aircall number, we need the transfer to display the user’s phone number (the person we called) so Aircall can route the call to the correct advisor via caller ID lookup.

When we select “User’s Number” under Displayed Caller ID in the Transfer Call node, the transfer fails with: SIP status code: 403 — Caller ID is unauthorized. A Valid Caller ID is either a DID you have purchased from Twilio or a verified Caller ID.

When we select “Retell Agent’s Number,” the transfer works fine but Aircall sees the Retell number instead of the homeowner’s number, which breaks our routing.

Is this a known issue with outbound call transfers specifically? Is there a workaround or timeline for a fix?

Call ID for reference: call_e62f714b4f2a5415c18aebf24e8

2

Hi @baumberger.hans

This is a Twilio restriction, not a Retell bug. The 403 error occurs because Twilio only allows caller ID spoofing with numbers you own (purchased DIDs) or verified caller IDs in your Twilio account. When you select “User’s Number,” Retell sets the from and P-Asserted-Identity SIP headers to the user’s number — but Twilio rejects it since that number isn’t yours.

Thank You

3

Please reach out to me via Discord - I may be able to assist on this matter.