SIP trunking, explained for people buying voice AI
SIP trunking carries telephone calls over an internet connection rather than over traditional phone lines. It matters when buying voice AI because it determines whether an agent can be attached to the numbers you already publish, and because call minutes are often the largest line on the invoice.
- SIP trunking
- A way of carrying telephone calls over an internet connection instead of over dedicated phone lines, using the SIP protocol to set up and end each call. It is how a software voice agent gets attached to real phone numbers.
What it is
A telephone call has to travel somehow. Traditionally it travelled over dedicated phone lines rented from a telecoms company. SIP trunking carries it over an ordinary internet connection instead, using a protocol called SIP, defined in RFC 3261, to set up, manage and end each call.
The word “trunk” is inherited from the physical bundle of lines it replaced. In practice it is a connection between your phone numbers and whatever software answers them.
Why it comes up when buying voice AI
A voice agent is software. For it to answer your main number, that number’s calls have to reach the software, and SIP is usually how. Three practical consequences:
- Your existing numbers can usually stay. You do not have to publish a new number or reprint anything. The number keeps ringing where it rings; a copy of the call reaches the agent.
- Somebody is billing you per minute. Call minutes are frequently the largest line on a voice AI invoice, and they are often billed by a different company from the one selling you the agent. Ask which, and ask for both numbers before you compare vendors.
- How many calls at once has a limit, and the limit is a setting rather than a rebuild. Worth agreeing before a campaign rather than during one.
The two arrangements
The vendor supplies the numbers. Simplest to start, and everything appears on one invoice. You are then buying your telephony from your software vendor, which is fine until you want to leave.
You bring your own carrier. You keep your existing telecoms provider and the vendor connects to it. Usually cheaper at volume, and it keeps the number relationship in your name, which is the part that matters if you ever change software.
What to ask before signing
- Can my existing published numbers reach the agent without being ported?
- Who bills the call minutes, and at what rate?
- How many calls can run at once, and what happens at the limit?
- If I leave, do I keep my numbers, and how long does it take?
Questions this raises
Do I need to understand this to buy voice AI?
Not in detail. You need to know two things: whether your existing numbers can point at the agent without being moved, and who is billing you for the call minutes. Everything else is your telecoms provider’s problem and your vendor’s.
What does "bring your own carrier" mean?
That you keep your existing phone provider and the voice AI vendor connects to it, rather than making you buy numbers and minutes from them. It usually costs less and it always makes leaving easier, which is why it is worth asking about before you sign.
What is call trunking?
Call trunking is the everyday name for SIP trunking: a trunk is the connection that carries many phone calls at once between your phone numbers and the software or provider that answers them. Over an internet connection it replaces the dedicated phone lines the word was borrowed from.
Sources
- RFC 3261 — SIP: Session Initiation Protocol · checked 2026-09-02
- Voicely internal: project.md — telephony over a CPaaS provider · checked 2026-09-02