Hello Support Team, 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>

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>

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.

Thank you for the detailed description. To help us provide the most accurate guidance, could you please confirm whether you are using a "BYO SIP trunk" configuration in Vapi, and if so, have you already specified the gateway IP and port (e.g.,

5080

) in your trunk setup? Sources: - [Vapi SIP Trunk Documentation](https://docs.vapi.ai/advanced/sip/sip-trunk) - [Vapi SIP Networking Reference](https://docs.vapi.ai/advanced/sip/sip-networking) - [Vapi SIP Trunk Troubleshooting](https://docs.vapi.ai/advanced/sip/troubleshoot-sip-trunk-credential-errors)

Yeah, you’re right to flag this, Karel usually relies on consistent From/To and domain matching, so Vapi’s default sip.vapi.ai format can break call tracking. From experience, Vapi doesn’t give much control over REGISTER or header rewriting, so people typically fix this with a SIP proxy (like OpenSIPS/Kamailio) in between. That also helps handle the port in the Request-URI and adding Route headers cleanly. Would you be open to using a SIP proxy, or are you trying to keep it as a direct connection only? @Tudoshiy

hey mate i want to make it with proxy , could you please help me how to do that

Hi Tudoshiy, Thank you for the detailed breakdown. Here are answers to each of your questions: 1. Can the REGISTER / SIP signaling format be customized? Partially. When creating a BYO SIP Trunk credential, you can configure a

sipRegisterPlan

under

outboundAuthenticationPlan

with

domain

,

username

, and

realm

fields to influence the REGISTER message format. SIP signaling will continue to follow the

<phone_number>@<credential_id>.sip.vapi.ai

format 2. Can the Request-URI include a custom port like `:5080`**?** Yes. Set

port: 5080

on your gateway object in the

gateways

array of your BYO SIP Trunk credential. This is fully supported (range 1–65535, default 5060). 3. Can From / To / Contact headers be rewritten to match your trunk domain? The

sipRegisterPlan.domain

field is designed for this purpose. Setting

domain: "tolunaydin.connect.com"

along with the appropriate

username

and

realm

should influence the From/To headers sent in the REGISTER. 4. Is adding a Route header supported? Not currently. This is not an available configuration option at this time. 5. Is there a recommended configuration for Karel PBX? We don't have Karel-specific guidance. We recommend configuring your BYO SIP Trunk with the gateway

port

set to

5080

, and using the

sipRegisterPlan

fields (

domain

,

username

,

realm

) to align the REGISTER headers with what Karel expects. If any of these capabilities don't meet your needs or you'd like to request enhancements (such as Route header support), please submit a feature request here: https://roadmap.vapi.ai/feature-requests Thank you for reaching out.