Hi, We're trying to set up a byo-sip-trunk with a UK-based SIP provider, but we're running into a registration failure and cannot get VAPI to work with this trunk. The Problem: We're using the POST /credential endpoint with the byo-sip-trunk provider type and including a sipRegisterPlan. Our SIP provider has confirmed they are receiving our REGISTER attempts, but they are rejecting them with the SIP error code: 476 No Server Address in Contacts Allowed. The Cause (Confirmed by Provider): Our provider has shared a SIP trace (see attached image) showing that Vapi is sending the Contact header in the REGISTER message like this: Contact: <sip:OUR_SIP_USERNAME@sip.vapi.ai;transport=udp...etc Their platform policy requires the Contact header to use the source IP address of the registering client (Vapi's IP), not sip.vapi.ai. They expect to see: Contact: <sip:OUR_SIP_USERNAME@VAPIS_SENDING_IP;transport=udp...etc Our Question: We've reviewed the CreateByoSipTrunkCredentialDTO schema and can't find an explicit field to control the domain/host used within the Contact header during registration. Is there any way, via the API payload for example, to configure the byo-sip-trunk to use the source IP address (or another specific value) in the Contact header instead of sip.vapi.ai to meet our provider's requirement? We've confirmed we're using the correct sipRegisterPlan structure (domain, username, realm) and the correct SIP host provided by our provider. The Contact header seems to be the final hurdle. (Will upload the SIP trace image contact header field) I have also posted an email support ticket with more detail. Any help or insight would be greatly appreciated! Thanks. https://cdn.discordapp.com/attachments/1376912496199205004/1376912496421634048/vapi-contact-header-discord-image.png?ex=68370d30&is=6835bbb0&hm=c994872e50c6bef945b9016a56e807d6d845c7c175f4aba7538c8e11c413aed7&

Currently, there is no supported way in the Vapi API or configuration to customize the domain/host in the Contact header of SIP REGISTER messages for BYO SIP trunks. The Contact header will default to

sip.vapi.ai

, and there is no documented field to override this to use the source IP address. If this is a critical requirement, please continue working with Vapi support, as this may require a platform-level change or feature request. Sources: - [Vapi SIP Trunking Documentation](https://docs.vapi.ai/advanced/sip/sip-trunk) - [Vapi API Reference: Create Credential](https://api.vapi.ai/api)

Message marked as helpful by @sammysammyboi! 🎉

@sammysammyboi I can help you resolve this issue, via the Cloudonix SBC infrastructure. If you would like to test and experiment, please DM me.

Currently, there is no supported way to customize the domain/host in the Contact header of SIP REGISTER messages for BYO SIP trunks in Vapi. The Contact header will always use

sip.vapi.ai

, and this cannot be changed via the API or configuration. If this is a critical requirement for your provider, please continue working with Vapi support to request this as a platform feature. For reference, see the [Vapi SIP Trunking Documentation](https://docs.vapi.ai/advanced/sip/sip-trunk). Sources: - [Vapi SIP Trunking Documentation](https://docs.vapi.ai/advanced/sip/sip-trunk) - [Vapi API Reference: Create Credential](https://api.vapi.ai/api)

@Nir S (CEO/Founder @Cloudonix) Thanks for your offer, although I’d like to politely decline.

That's fine, I can only offer assistance, that's all.

Given the current platform limitations, submitting a feature request is the most direct way to address the need for customizing the Contact header in SIP REGISTER messages. Since there isn't a supported method or configuration to achieve this within Vapi right now, a feature request would be the recommended route if this requirement is essential for your use case. If you prefer not to wait for a platform change, exploring external solutions or workarounds outside of Vapi might be necessary, but within the platform itself, customization isn't currently possible.

As we already discussed in OH Sammy, can you create a ticket in jambonz and ask over there?

Hey Sahil, sorry for the delay in getting back to you

> What would getting a ticket setup there achieve? If we can't change it on the vapi side and a new feature isn't there to allow that to happen then theres not much that would get done by doing that? Hey @sammysammyboi appreciate your thoughts, You can raise an feature request for it over here: and then you can follow up with team.