In our SIP REFER transfer scenario, Vapi acts as the transferor/referrer. The original caller A calls our Vapi BYO number B, then the Vapi assistant B initiates a blind transfer to C by sending a SIP REFER.

Our scenario in detail:

2026-07-01 10:48:28 CET

A (original caller): 36705135714

B (vapi): 3615060968

C: 3615060954

And the REFER header sent by vapi:

From: <sip:3615060968@192.168.21.55>;tag=t7caKBFytDFcg

To: "36705135714" <sip:36705135714@192.168.21.1>;tag=as0780c1b7

Refer-To: <sip:0615060954@91.223.222.55:5060>

Referred-By: "36705135714" <sip:36705135714@172.31.9.106>

Call-ID: 34ac5c96085fd77c00bd363156006388@192.168.21.1:5060

According to RFC 5589, the party initiating the transfer is the Transferor, and the REFER is issued by the Transferor to the Transferee. According to RFC 3892, the Referred-By header carries the identity of the referrer.

Therefore, in this scenario, the Referred-By header should contain the Vapi/BYO number identity B, but it should not contain the original caller A. Currently Vapi sends the original caller A in Referred-By, which breaks our SIP transfer authorization logic.

Please confirm whether Vapi can either: