We are currently trying to integrate Vapi with a Karel PBX through a SIP trunk, and we are facing an issue related to SIP REGISTER/signaling format compatibility. When we inspect the SIP REGISTER message generated from the Vapi side, it is currently being sent in a structure like this: *

REGISTER sip:tolunaydin.connect.com SIP/2.0

*

Via: SIP/2.0/UDP 34.213.129.25

*

From: <sip:1000@fe791a92-81a7-4b7a-a33a-9f13acb964fe.sip.vapi.ai>

*

To: <sip:1000@fe791a92-81a7-4b7a-a33a-9f13acb964fe.sip.vapi.ai>

*

Call-ID: ...

*

CSeq: ... REGISTER

However, for Karel to properly track and associate the active call, the SIP signaling needs to be in a structure closer to the following format: *

REGISTER sip:tolunaydin.connect.com:5080 SIP/2.0

*

Via: SIP/2.0/UDP 10.40.44.13:54548

*

Route: <sip:test1connect.karel.cloud:5080;lr>

*

From: "1000" <sip:1000@tolunaydin.connect.com>

*

To: "1000" <sip:1000@tolunaydin.connect.com>

*

Call-ID: ...

*

CSeq: ... REGISTER

The main concern is that the current Vapi-generated REGISTER message uses the

sip.vapi.ai

domain in the From/To headers and a different Contact format, while Karel expects the registration and signaling to be associated with our trunk/domain structure in order to correctly monitor and match the active call. Could you please confirm: 1. Whether the REGISTER / SIP signaling format can be customized on the Vapi side 2. Whether the Request-URI can include the required port, such as

:5080

3. Whether the From / To / Contact headers can be rewritten to match the trunk domain 4. Whether adding a Route header is supported 5. Whether there is any recommended configuration for compatibility with Karel PBX systems This signaling/header compatibility is critical for Karel to properly track and associate active calls. Thank you in advance for your support.