VAPI•9mo ago

3CX ↔ Vapi via Twilio – Two-way SIP setup questions

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!

Vapi Support BotAPP•4/11/25, 9:33 AM

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()transferCall() function, but there are some important considerations. According to the documentation, the Transfer Call task can redirect active calls to external phone numbers. However, Vapi doesn't support PSTN to SIP transfers, 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.

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 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 when using Twilio.

Known limitations

There are several known limitations:

Caller ID issues: Calls can sometimes be blocked due to caller ID problems, and some users have reported that "all call IDs are same."

Vapi Support BotAPP•4/11/25, 9:33 AM

Latency: There can be high latency when connecting to Vapi servers, which affects call quality. Using Vapi's infrastructure in the closest region to your location can help reduce this.
Audio quality: Audio quality can be affected by codec incompatibility between systems.
Voicemail detection: Vapi sometimes returns 404 errors to Twilio's AMD requests, leading to missed voicemail detection.

### Best practices
Here are some best practices for your setup:

SIP Configuration: Verify your network configuration allows SIP traffic, ensure your SIP server accepts calls from Vapi's servers, and confirm your system handles SIP REFER messages properly.
Authentication: If using authentication, double-check credentials and consider using Twilio API keys with restricted permissions for enhanced security.
Setup order: Configure SIP Trunking first before setting up call forwarding.
Monitoring: Use Twilio's call quality reports and analytics to track performance and troubleshoot issues.

### Alternative integration methods
Yes, there are alternatives to using Twilio:

Vapi Support BotAPP•4/11/25, 9:33 AM

Direct SIP Connection: This might be possible if Vapi supports proxy authentication headers, which 3CX requires.
Alternative SIP Providers: You could use other providers like Sonetel, Telnyx, voip.ms, or zoom.com for SIP trunking.
API-Based Call Handoff: Vapi can manage the call and send caller details via API to your system.
WebRTC or REST API: These can be used to connect your PBX with Vapi.
Asterisk: This can be used as an alternative to Twilio for cost reasons.

Is there anything specific about any of these aspects you'd like me to elaborate on?

Sources:

Vapi Support BotAPP•4/11/25, 9:33 AM

⤵️ Mention @Vapi for follow-ups 🐥

verbal-limeOP•4/11/25, 9:53 AM

@Vapi Support Bot @Vapi

Vapi Support BotAPP•4/11/25, 9:53 AM

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?

VapiAPP•4/11/25, 8:15 PM

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.

VapiAPP•11/18/25, 4:32 AM

I sincerely apologize for the delayed response to your support request.
We implemented better tracking for support tickets to ensure timely responses going forward, but due to a technical limitation, this only applies to tickets created after this past week.
I'm closing this older ticket, but if your issue still needs attention, please:
Reply with 'REOPEN' or
Email support@vapi.ai with any updates
I'll personally ensure your new request gets prompt attention.
Thank you for your patience.
Best regards,
Vapi Support Team

3CX ↔ Vapi via Twilio – Two-way SIP setup questions

Can Vapi redirect calls back to 3CX?

Reliability of two-way SIP flow

Known limitations

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