Setting up a SIP trunk for Voice AI: step-by-step (Twilio + Vapi)

SIP trunk configuration is the part of every Voice AI project that looks simple in the documentation and takes three times longer than expected in practice. Not because the technology is hard - it is not - but because the failure modes are silent, the error messages are cryptic, and the gap between a correct-looking configuration and a working one is often a single codec setting. This is the step-by-step guide I use on every deployment. It covers Twilio as the SIP provider and Vapi as the Voice AI platform - the most common combination in production. What you need before starting A Twilio account with credit added A Vapi account (free tier is fine to start) A phone number purchased in Twilio (~$1/month) Access to your firewall settings if on a corporate network Step 1 - Create a SIP trunk in Twilio In Twilio Console go to: Voice → SIP Trunking → Trunks → Create new SIP Trunk Name it something descriptive like voice-ai-production . Settings to configure: Call Recording: Record from Ring