I have a voice AI agent in Vapi making outbound calls using a Plivo SIP Trunk number. The outbound call works well. However, when I the agent tries to transfer the call, the call drops. I am using "sipUri": "sip:1XXXX@XXXX.plivo.com" which works well for other SIP providers. I worry that the issue is that Plivo does not support SIP REFERs https://support.plivo.com/hc/en-us/articles/360041475971-What-SIP-request-methods-does-Zentrunk-SIP-trunking-support and that's what Vapi is using. Is there a workaround here? In other words, has anyone been able to transfer calls from agents connected to Plivo SIP numbers?

Plivo SIP trunks do not support SIP REFER, which is required for call transfers in Vapi. As a result, any attempt to transfer calls from a Plivo SIP number will drop the call—this is a documented limitation, not a configuration issue. There is currently no documented workaround for enabling call transfers with Plivo SIP trunks in Vapi. For reference, see the official [Vapi Plivo SIP documentation](https://docs.vapi.ai/advanced/sip/plivo). Source: - [Vapi Plivo SIP Integration Documentation](https://docs.vapi.ai/advanced/sip/plivo)

Message marked as helpful by @manudo! 🎉

Hi manudo, You're correct — Plivo Zentrunk does not accept SIP REFER, and Vapi's transfer flow for BYO SIP trunks relies on REFER. The Plivo guide in our docs confirms this limitation: https://docs.vapi.ai/advanced/sip/plivo Workarounds/options: • Use a different SIP provider that supports REFER (Twilio, Telnyx with proper config) and keep your Plivo number moved there. • Use a PSTN provider (Twilio) for numbers used with warm transfers — warm-transfer features are only fully supported on Twilio. • Implement proxying: have your own SIP gateway/B2BUA that accepts Vapi's REFER and re-originate an authenticated outbound call via Plivo using the provider credentials. (This requires building/hosting a SIP B2BUA and is non-trivial.) • As a different approach, do the transfer from your telephony provider side: have Vapi signal a webhook or server message so your backend can initiate the outbound transfer using Plivo’s authenticated outbound trunk (if Plivo accepts such an API/Call Control flow). Note: Plivo still must support the method used; many customers report no viable REFER-based transfer via Plivo. Short answer: there is no simple documented workaround inside Vapi — the practical options are switching provider for transfer-enabled numbers or implementing a SIP proxy/B2BUA to bridge Vapi ↔ Plivo with authenticated outbound calls.