It seems Vapi supports "native" SIP trunk integrations with Plivo, Telnyx and Zadarma. However, we are trying to integrate to a SIP Trunk from Twilio (it must be the case, we can't use Vapi's Twilio native integration). The SIP integration for making and receiving calls works well, but we can't get the call transfer to work. Despite having Call Transfer enabled in the Twilio SIP Trunk we are integrating with, it seems that Vapi is not sending the REFER packet to Twilio as we don't see a child call or the packet itself in the call's PCAP log. Does anyone know everything about this? Does Vapi send REFER packets in BYO SIP integrations? How should it be enabled?

Thanks for the detailed context! To help pinpoint why REFER packets aren't being sent, could you confirm if your Vapi assistant's configuration has

phoneCallProviderBypassEnabled

set to

false

and that there are no custom

serverMessages

(like

"phone-call-control"

) overriding default call control? This is a common cause for REFER not being triggered in BYO SIP setups. If you can share this, I can guide you on the next troubleshooting step! Sources: - [Vapi documentation: call forwarding](https://docs.vapi.ai/call-forwarding) - [Vapi SIP Trunking documentation](https://docs.vapi.ai/advanced/sip/sip-trunk) - [Debug call forwarding drops](https://docs.vapi.ai/phone-calling/transfer-calls/debug-forwarding-drops)

Changing phoneCallProviderBypassEnabled to false solved this. We did not have it set as true, just not configured. Is is true by default?

Message marked as helpful by @manudo! 🎉

We’d like to check in to see if you need any further assistance. We’re sorry for the inconvenience and are more than happy to help if there’s anything else you need.

Yes, I have another question rrelated to this topic. If I am configuring a transfer call tool to transfer to a SIP number in the form of "sip: [number]@example.sip.com" from a phone number used by the agent that is "[another_number]@differentexample.sip.com", basically a different domain, is this feasible? all within the same telephony provideer

Yes, it is feasible to transfer a call from one SIP domain to another SIP domain, even when they have different domains (like transferring from " Here's what you need to know: SIP Call Transfer Between Different Domains 1. **Transfer Call Configuration**: You can transfer a call to a SIP URI in a different domain using the transfer call tool:

{
  "type": "transferCall",
  "destinations": [
    {
      "type": "sip",
      "sipUri": "sip:[number]@example.sip.com"
    }
  ]
}

2. **SIP REFER Requirements**: This process uses the SIP REFER protocol, which needs to be enabled in your telephony provider. The provider handles the routing between different domains as long as they're within their network. [1](https://docs.vapi.ai/advanced/sip/sip-trunk) 3. **Format Requirements**: Some providers require phone numbers to be formatted in E.164 standard, so your SIP URI might need to look like:

sip:+1XXXXXXXXXX@example.sip.com

4. **Provider Consistency**: The transfer works most reliably when both domains are within the same provider's network. The telephony provider manages the actual SIP REFER handling after Vapi initiates the transfer. When setting this up, make sure your telephony provider supports SIP REFER between different domains within their network, as this capability varies by provider.