@User Zadarma Answer: Dear Juan Ayensa Barbarin, Your ticket is already being processed by the technical department. The calls shown in your examples appear to be one incoming call and one outgoing call, both processed correctly on our side. We did not receive any transfer attempt, meaning SIP REFER packets, for either of the two calls. These two calls should probably have been merged by Vapi. You can ask Vapi support: why were they not merged? You can also view and request the SIP traces in Vapi. As you mentioned earlier, “warm transfer” is an experimental Vapi feature. To learn more about how to use this experimental feature, we strongly recommend contacting the Vapi support team for any questions related to that functionality. https://cdn.discordapp.com/attachments/1513928341831549088/1513928342338797699/message.txt?ex=6a298324&is=6a2831a4&hm=2085f27361fd813eb310040398ce4a1ed73845bbdf72c395ea6ca6d179b57467&

Hi Juan, Thank you for the thorough testing and for including the call IDs - that made a precise investigation possible. I have fully traced both calls. The outbound test works correctly because both call legs go through the same internal session layer. The inbound test hits a different path: when a call arrives through your Zadarma SIP trunk, it is handled by a different internal session account than the one Vapi uses to dial the outbound screening call. When warm-transfer-experimental tries to bridge those two into a conference room, it creates two separate rooms that cannot connect to each other. Both you and Débora are placed in silence, and the call ends without the bridge forming. This is a limitation in how warm-transfer-experimental currently handles inbound calls on BYO SIP trunks. It is not a configuration issue on your side, and Zadarma has confirmed their infrastructure processed both legs correctly. I have escalated this to our engineering team with the full technical trace. I will update you as soon as I have information on a fix or a recommended workaround. Your config and the way you have built the screening flow are well-structured. Once the underlying session routing is resolved, it should work exactly as intended. Regards, Chiranjeet Vapi Support

Oks at the momment i will use blind transfer with some work around that i have made and once you give some work around or you solve de session issue i will try again to introduce it to my Vapi agents. Thaks for the answer

Hi Juan, That sounds like a sensible approach. Blind transfer will serve you reliably in the meantime. I will keep this ticket open and reach out as soon as there is a resolution or workaround for the inbound session issue. Thanks for your patience and for the quality of the testing you shared, it made the root cause much faster to find Regards, Chiranjeet Vapi Support

Okay, thank you very much. I’ll stay tuned for any future updates from your side.