Hello there, I’m currently working on connecting our 3CX system to Vapi using Twilio as a SIP bridge, as recommended here and on the inbound SIP guide. It’s working well in one direction (3CX → Twilio → Vapi), but before I try this setup on our real 3CX, I just have a few questions: - Can Vapi redirect a live call back to another 3CX number via transferCall()? - Does the Twilio SIP bridge support this two-way flow reliably? (Vapi → Twilio → 3CX) - Are there any known limitations (caller ID, audio quality, latency) with this setup? - Any best practices or examples you’d recommend? - And finally, is using Twilio the only current way to achieve this kind of integration with 3CX? Thanks a lot in advance 🙏 Happy to share more details/screenshots if needed!

Hi there! 👋 I'd be happy to help with your questions about connecting 3CX to Vapi using Twilio as a SIP bridge! Regarding your questions: ### Can Vapi redirect calls back to 3CX? Yes, Vapi can redirect calls using the

transferCall()

function, but there are some important considerations. According to the [documentation](https://docs.vapi.ai/workflows/tasks/transfer), the Transfer Call task can redirect active calls to external phone numbers. However, Vapi [doesn't support PSTN to SIP transfers](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1334919245468799007), but does support SIP to SIP transfers. Since you're using Twilio as a SIP bridge, this may allow Vapi to redirect calls back to a 3CX number, though some users have reported [issues with the transferCall() function](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1338538066733957152). ### Reliability of two-way SIP flow The two-way SIP flow (Vapi → Twilio → 3CX) can work but has some reliability challenges. Users have reported [intermittent call failures](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1341449578100031488) with SIP forwarding to Vapi, including 491 errors occurring about 20% of the time. There have also been instances where [Vapi doesn't consistently answer calls](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1343208312249913397) when using Twilio. ### Known limitations There are several known limitations: 1. **Caller ID issues**: Calls can sometimes be [blocked due to caller ID problems](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1341213363744800769), and some users have reported that "all call IDs are same."

@Vapi Support Bot @Vapi

Hi there! 👋 I see you've tagged me. Is there something specific about the 3CX to Vapi integration via Twilio that you'd like me to clarify or expand on? I'm happy to provide more details on any of the topics we discussed earlier or answer any new questions you might have. Is there anything else I can help with regarding your SIP bridge setup?

I am about to answer all of your questions in the order of ask. The answer to the first question is yes, Vapi can transfer a call to any number, whether it's a phone number or a SIP number. Regarding that Twilio call or SIP bridge, you have to check with them. As of now, there is no known limitation on our side, but it's good to check with Twilio and 3CX. I'll suggest checking Cloudinix blogs for best practices and recommendations. Twilio is not the only way; you can even use Cloudinix as an alternative to it.