Connect Your AI Voice Agent to a UK 0800 Number
You have built an AI voice agent. It understands natural language, it can book appointments, qualify leads, and handle tier-1 support queries without a human in the loop. There is just one problem: it is sitting in the cloud with no way for your actual UK customers to reach it by phone.
That is exactly what we solve. We provide the SIP trunk that bridges the gap between the UK public telephone network and your AI platform. When a customer dials your 0800 Freephone number, we route that call straight into your Vapi agent, your ElevenLabs conversational AI, your Retell bot, or whatever platform you have built on.
Customer dials your 0800 number → Our SIP trunk receives the call → We forward it as a SIP INVITE to your AI platform's endpoint → Your AI agent picks up and starts talking. The caller has no idea they are speaking to an AI unless you want them to know. The entire handoff takes under 200 milliseconds.
The Call Flow: PSTN → SIP Trunk → AI Agent
Here is exactly what happens, step by step, when a UK customer calls your AI-powered 0800 number:
The critical piece that most AI builders overlook is step 3. Your Vapi agent cannot just "listen" to the UK phone network on its own. It lives in a cloud data centre. It speaks SIP and WebRTC, not SS7 (the protocol used by traditional phone carriers). You need a SIP trunk provider that sits between the public telephone network and your AI platform, converting PSTN calls into SIP sessions that your agent can understand.
We are that provider. And because we give you a UK 0800 Freephone number as the front door, your customers call for free — which means significantly higher pickup rates than if you were using a standard mobile or geographic number.
Compatible AI Voice Platforms
Our SIP trunks work with any platform that can receive an inbound SIP connection. If your platform documentation mentions "SIP URI," "SIP endpoint," or "BYOC (Bring Your Own Carrier)," it is compatible. Here are the platforms we see most frequently:
Vapi
The most popular AI voice agent builder. Supports inbound SIP natively via their Phone Numbers dashboard.
SIP Compatible ✓ElevenLabs
Industry-leading voice synthesis. Their Conversational AI product accepts SIP trunk connections for phone-based agents.
SIP Compatible ✓Retell AI
Purpose-built for phone calls. Retell's BYOC feature lets you bring your own SIP trunk and map it to an agent.
SIP Compatible ✓Bland AI
Enterprise-focused AI calling platform. Supports SIP interconnection for high-volume inbound deployments.
SIP Compatible ✓Cognigy
Enterprise conversational AI with deep contact centre integration. Full SIP trunk support via Voice Gateway.
SIP Compatible ✓NICE CXone
One of the world's largest CCaaS platforms. Accepts external SIP trunks for AI-augmented voice workflows.
SIP Compatible ✓Not listed? If your platform accepts SIP, it works. We have also connected 0800 numbers to custom-built agents running on Voiceflow, Synthflow, LiveKit, and raw Twilio/Vonage SIP bridges. The protocol is standardised — there is nothing proprietary about our trunks.
Need a SIP Trunk for Your AI Agent?
Get a UK 0800 number with SIP forwarding. Live in under an hour.
Book a Technical Setup CallWhat is SIP Trunking? (The Plain English Version)
If you are a developer, you already know what SIP is. If you are a business owner who hired someone to build your AI agent, here is the non-technical explanation:
Think of a SIP trunk as a digital phone cable. In the old days, your office had physical copper phone lines running from BT's exchange into your building. Each line was a "trunk." If you wanted 10 simultaneous calls, you needed 10 physical trunks bolted into your wall.
A SIP trunk does the same thing, but over the internet. Instead of copper wires, the call data travels as packets across your internet connection. Instead of terminating at a desk phone, the call terminates at a software endpoint — which, in your case, is your AI voice agent sitting in AWS, Google Cloud, or Azure.
The practical upside: you can scale from 1 concurrent call to 500 concurrent calls without anyone touching a single piece of hardware. You just tell us how many channels you need, and we provision them instantly.
Technical Setup: Connecting Your AI Agent
The actual configuration is straightforward. Once you purchase an 0800 number from us, you configure call forwarding to point at your AI platform's SIP endpoint. Here is a simplified example of the data you would provide:
# Example: Forwarding an 0800 number to a Vapi SIP endpoint
Number: 0800 XXX XXXX
Forward To: sip:your-agent-id@sip.vapi.ai
Protocol: SIP/UDP or SIP/TLS
Codecs: G.711 (μ-law), Opus
Concurrency: 10 channels (Business) / Unlimited (Enterprise)
# Example: Forwarding to a Retell AI BYOC endpoint
Number: 0800 XXX XXXX
Forward To: sip:your-agent@byoc.retellai.com
Protocol: SIP/TLS (recommended)
Auth: IP Whitelisting or Digest Auth
If this looks unfamiliar, do not worry. Our technical team will walk you through the entire setup during a 15-minute onboarding call. Most integrations are live and taking calls within an hour.
Why Use an 0800 Number Instead of a Standard Number?
You could technically connect your AI agent to any phone number. A standard UK mobile, a geographic 020 London number, or a 0333 number. So why specifically an 0800?
Scalability for Enterprise AI Deployments
If you are deploying AI voice agents at scale — handling thousands of concurrent calls across multiple departments, languages, or geographic regions — our Enterprise SIP trunking tier is built specifically for you.
Enterprise clients get:
Deploying AI Voice at Scale?
Talk to our enterprise team about dedicated SIP trunking, custom SLAs, and volume pricing.
Contact Enterprise Sales View Standard Pricing