What is a SIP trunk: a plain explanation for business, a channel estimate and how to connect one

Sado AI team · November 5, 2026 · 5 min read

In short

A SIP trunk is a line from your telephone provider to your company’s system (an office PBX, a cloud PBX or an AI operator) over the internet. One trunk carries several channels, and each channel is one call at a time. One or more numbers are attached to it. To connect, the provider gives you a login, password and server address, or adds your IP address to its allowed list. Work out how many channels you need from your busiest hour, then test the trunk with a few calls.

SIP trunk vs a single SIP line

A plain SIP account is one line for one phone or one app, such as the IP phone on a cashier’s desk. A SIP trunk is a wider pipe: one connection comes from the provider, several calls run inside it at once, and your own system routes each call to the right person or operator.

Each channel in the trunk is one call at a time. With a four-channel trunk, four people can talk at once and the fifth caller hears a busy tone. Incoming and outgoing calls share the same channels.

Who needs a trunk

You have a mini-PBX or a cloud PBX, and calls are shared between several staff members.
One number gets more than two or three calls at the same moment: during ads, promotions or the busy season.
You have several numbers (for example, separate ones for sales and service) and want to manage them in one place.
An AI operator answers your calls: it also connects over SIP and takes several calls at once.

A small shop with a handful of calls a day on one mobile number doesn’t need a trunk; a plain line is enough.

What to ask your provider

How many channels you get and what an extra channel costs.
Which numbers are attached to the trunk: city or mobile, one or several.
How it connects: by login and password, or by IP address. The second needs a fixed (static) IP.
Which number the customer sees when you call out.
Which codecs work (for example G.711 or G.729): your system must support them too.
Per-minute prices: city, mobile, other regions, abroad; and whether incoming calls are charged.
Where to report a fault and how fast they respond.

How many channels: a simple estimate

Take your busiest hour: calls in that hour × average length in minutes ÷ 60. The result is the average number of calls running at the same time in that hour. Calls don’t arrive evenly, so allow roughly twice that.

For example, the furniture shop Uy Mebel gets 24 calls between 11 and 12 on Monday, 4 minutes each: 24 × 4 ÷ 60 = 1.6. On average two calls run at once, but in some minutes three or four overlap. Four channels are enough for this shop. If you call customers back in the morning, add those outgoing calls too. These numbers are only an example.

What to test after connecting

Incoming: call your number from another phone and check that both sides hear each other.
Outgoing: call a mobile and a city number from the system and see which number shows on the screen.
At the same time: call from several phones at once and check that you have the channels you ordered.
A long call: talk for 10–15 minutes. If calls always drop at the same second, the router settings are the usual cause.
Nights and weekends: check where calls go, so none end up where nobody answers.

Connecting an AI operator to the trunk

Sado’s AI operator also connects over SIP: your provider (for example Sarkor or UTeL) gives a login, password and server address for your number, and Sado’s team does the rest of the setup. Your number stays the same, and outgoing calls come from it too. The operator takes more than ten calls at once, but how many calls pass through one number depends on the trunk’s channels, so the channel estimate matters here too.

You choose which calls the operator answers: all of them, or only after hours. More: AI operator for incoming calls, connecting your own number and questions about Sado. Try it live or call 78 778 01 23.

Questions

Does a SIP trunk need good office internet?

If the trunk comes into an office PBX, yes: calls run over the office connection, so you need a stable wired line. If the provider connects the trunk straight to a cloud system or an AI operator, calls don’t pass through the office.

Is a SIP trunk the same as IP telephony?

No. IP telephony is the general name for calls over the internet. A SIP trunk is the multi-channel connection between the provider and your system within it.

Can channels be added later?

Usually yes: the channel is added on the provider’s side and the number stays the same. Ask your provider what it costs and how quickly it’s done.