A phone system on its own can only connect people inside your company. To call a customer or receive a call on your main number, it needs a connection to the public phone network. For decades that was a bundle of copper or ISDN lines plugged into the PBX. Today it's usually a SIP trunk: the same job, delivered over the internet.
What a SIP trunk does
SIP (Session Initiation Protocol) is the standard that internet phone systems use to set up calls. A SIP trunk is a service from a provider that:
- gives you phone numbers — local, national or toll-free, depending on the provider and country;
- delivers incoming calls to those numbers to your PBX;
- carries outgoing calls from your PBX to any phone in the world;
- bills you for the numbers and the calls.
Your PBX does everything in between: which extension rings, which voice menu plays, which caller ID goes out. The trunk is the road; the PBX is the traffic control.
SIP trunk vs a traditional phone line
| Traditional line (analogue, ISDN) | SIP trunk | |
|---|---|---|
| Delivered over | Dedicated copper to your building | Your internet connection |
| Capacity | Fixed by the physical lines | Set by the number of channels you buy |
| Numbers | Tied to the line and often the location | Can come from several areas or countries |
| Works with | A PBX in your building | A PBX anywhere, including in the cloud |
Because a SIP trunk isn't tied to your building, it's the natural partner for a cloud PBX: the trunk connects to the PBX in the data centre, and your team connects from wherever they are.
Channels: how many calls at once
A SIP trunk's capacity is measured in channels — one channel per simultaneous call, in or out. A trunk with 4 channels can carry 4 calls at the same time; the fifth caller hears a busy signal, or the call goes to a failover you set up.
To estimate how many you need, think about your busiest moment, not your headcount. Count the calls your team makes and takes at the same time at peak, and add any callers waiting in a queue — they're calls in progress too. Some providers charge per channel; others include a number of channels in a plan.
Register, peer and account trunks
Yeastar P-Series, like most modern PBXs, supports more than one way of connecting a trunk. You'll meet these terms when you set one up:
- Register trunk: the PBX logs in to the provider with a username and password, like a phone logging in to a PBX. The most common type for small and mid-sized companies.
- Peer trunk: the provider and the PBX recognise each other by IP address, no login. Common with larger providers and carrier connections.
- Account trunk: the other way round — the provider's system logs in to your PBX with an account you create.
Your provider tells you which type to use and gives you the settings.
How to choose a SIP trunk provider
Look beyond the price per minute. A good fit:
- Offers numbers where you need them — your city, your country, other countries you serve — and can port your existing numbers in.
- Has clear pricing for numbers, calls to landlines and mobiles, international calls and extra channels.
- Handles emergency calls properly — ask how calls to 112 are routed and which address is registered for your numbers.
- Supports your PBX. Yeastar keeps a list of tested providers by country; many have a ready-made template on the PBX.
- Answers when something goes wrong — check what support you get and how to reach it.
- Offers fraud protection, such as spending limits or blocked destinations, so a compromised account can't run up an unlimited bill.
For a multi-country team, it's normal to use more than one provider — one per country — on the same PBX.
Connecting a SIP trunk to Yeastar P-Series Cloud
On a Yeastar P-Series Cloud PBX, you add a trunk in the admin panel. If your provider is on Yeastar's tested list, choose its country and name, and the template fills in the technical settings — you add the username, password and numbers from your provider. If it isn't, choose the general option and enter the settings your provider gives you. Then you create:
- inbound routes, which decide where calls to each number go;
- outbound routes, which decide which trunk each outgoing call uses.
Yeastar's documentation walks through each trunk type step by step.
On PBXfactory
We don't sell SIP trunks or phone numbers. You connect a trunk from the provider of your choice — or keep your current one — and number porting is done with that provider. That way you can pick the numbers and rates that fit your country. Setting up the trunk is part of configuring your PBX, which isn't included in our standard support; Yeastar's guides, your IT partner or our paid support packages can help. The PBX itself is €4.99 per seat per month or €54.99 per year, prices excluding VAT — see pricing.