Note: This article is in conjunction with the following articles:
- Understanding PBX for Voice AI,
- Overflow and After-Hours Voice AI Receptionist,
- Understanding Call Forwarding to Voice AI Agent.
- Guide and best practices for purchasing phone numbers for Voice AI.
Make sure to read these articles before reading the current article.
In this article, I will show you step by step how to purchase a phone number from a carrier like ‘Twilio’ and then use that number in Retell AI for your Voice agents.
Purchasing a Twilio number is not as simple as selecting a number and clicking the purchase button. It is quite a complicated process, especially for first-time users.
Understand SIP Trunking first.
Understanding SIP trunking is important because it’s the “bridge” that connects your Twilio numbers to whatever phone system or Voice AI platform you use for calls.
When you understand SIP trunking, you can route Twilio numbers into Retell, GoHighLevel, PBXs, or custom call flows in a flexible, scalable way instead of being locked into one vendor’s built‑in telephony.
It also lets you centralise your phone infrastructure (numbers, compliance, routing) in Twilio while still swapping or adding Voice AI platforms as needed, which is exactly what you want for long‑term control over your communications stack.
Basics of SIP Trunking.
In order to understand the basics of SIP Trunking, you need to get familiar with the following terms:
# Session Initiation Protocol (SIP) - It is the set of rules used to set up, manage, and end calls (voice and video calls) over the Internet.
# Trunk - It refers to a “virtual phone line over the internet that carries calls between your VoIP or SIP based phone system and the public switch telephone network (PSTN).
# SIP Trunk - It refers to the trunk that uses the Session Initiation Protocol (SIP).
# Think of SIP trunking as owning your phone line rather than renting one from someone else. When you own the trunk, you decide where calls go. Retell AI today. GoHighLevel tomorrow. Both simultaneously if needed.
# VoIP (Voice over Internet Protocol) phone system - This system allows you to make/receive voice and/or video calls using the internet, instead of traditional phone lines. It works by converting your voice (or video) into digital data packets that travel over IP networks.
# Examples of VoIP phone systems: RingCentral, Dialpad, Zoom etc.
# Public switch telephone network (PSTN) - It is the traditional phone network that lets people call any phone number in the world, whether from landlines, mobiles, or payphones.
# Elastic SIP Trunking - It means the SIP Trunking that is scalable and flexible on demand.
# Elastic SIP Trunking lets you:
- Instantly increase or decrease call capacity with software, no hardware changes.
- Pay only for the channels and numbers you use.
- Add new numbers or regions in minutes.
Note(1): You would need to enable elastic SIP trunking for a purchased number in Twilio before you can import that number into a platform like Retell AI.
Note(2): For a typical Voice Agent scenario, calls go to your Twilio number first, and Twilio then routes the calls to Retell AI via SIP trunk or webhook for interaction.
Why SIP Trunking matters for Voice AI:
- One programmable phone number can connect to multiple platforms.
- Full control over call routing, recording, and compliance.
- Proper client isolation through subaccounts.
- You're building on infrastructure you own.
- Clients can be offboarded cleanly without touching other accounts.
SIP Trunking matters even more for non-US/Canada phone numbers.
If you need a phone number in the UK, EU, India, Australia, or most regulated markets, you can't just buy it instantly from Retell. You need a Regulatory Compliance Bundle first.
It doesn't matter where you're based.
Building Voice AI agents for UK clients from the US? You still need an approved bundle for that UK number.
That means submitting government-issued ID, proof of address, and business registration. Then waiting days (sometimes weeks) for approval.
The approval process can take a few days to a couple of weeks. But once approved, the actual configuration takes about an hour, provided you know all the necessary steps beforehand.
I've written a complete walkthrough covering every configuration screen, naming conventions, and the exact settings to enable.