What is SIP trunking, and does your business need it?
Short answer: a SIP trunk is a phone line delivered over the internet that connects a phone system to the public phone network — each channel carries one call. You need trunks if you run your own PBX. With a hosted cloud phone system, the connection is normally included, so you buy lines, not trunks.
SIP trunking in plain English
A business phone system needs a way to reach the outside world — to call customers’ mobiles and landlines, and to receive their calls. That connection used to be physical phone lines from the exchange. A SIP trunk does the same job over the internet: it links your phone system to the public phone network.
A trunk is made up of channels. Each channel carries one call at a time, so a trunk with 4 channels can handle 4 calls at once. Your phone numbers are attached to the trunk.
When you need SIP trunks — and when you don’t
- You run your own PBX (an on-site system, or your own server in the cloud) → you need SIP trunks from a provider to make and receive calls.
- You use a hosted cloud phone system → the provider normally includes the connection to the phone network. You buy lines as part of the service, not trunks separately.
- You use Microsoft Teams for calling → Teams needs a Microsoft Calling Plan, Operator Connect, or Direct Routing to a SIP provider or phone system. See 3CX vs Teams Phone.
SIP trunk vs cloud phone system
A SIP trunk is one part of a phone setup — the outside connection. A cloud phone system is the whole thing: the system that routes calls (IVR menus, queues, ring groups, voicemail), the extensions and apps for every staff member, and the connection to the phone network. If you buy bare SIP trunks, someone still has to provide, configure and look after the phone system they plug into.
How many channels or lines do you need?
Count the most calls your business has going at the same moment — incoming and outgoing together — and add a little headroom. A common rule of thumb is about one line for every 3–4 staff who use the phone; reception-heavy or sales teams need more. This is the same logic behind per-line pricing: you pay for the calls you make at once, not for every person.
What to check before choosing SIP trunks
- Are calls included, or charged per call or per minute?
- Can you port your existing numbers, and is it free?
- Is there a lock-in contract?
- What happens to incoming calls if your internet goes down? (Look for automatic divert — see outages.)
- Who supports the phone system the trunks connect to?
The Wise Cloud Technology approach: each line is one simultaneous call — $29.95 ex GST per unlimited line per month, calls included, priced per line not per user — and it comes with the hosted 3CX phone system, apps for every staff member, free number porting, installation on site and local support. No lock-in contract, and nothing to pay until it’s working.
Quick answers
What is a SIP trunk?
A SIP trunk is a phone line delivered over the internet instead of copper. It connects a business phone system (PBX) to the public phone network, so the system can make and receive normal phone calls. Each trunk channel carries one call at a time.
Do I need SIP trunking for a cloud phone system?
Not as a separate purchase in most cases. With a hosted cloud phone system, the provider usually supplies the connection to the phone network as part of the service. You would buy SIP trunks yourself mainly if you run your own PBX, on site or in your own cloud.
What is the difference between a SIP trunk and VoIP?
VoIP is the general technology of carrying voice over the internet. A SIP trunk is one specific use of it: the connection between a phone system and the phone network. A cloud phone system uses VoIP for everything — the phones, the apps and the connection to the network.
Is a Wise Cloud Technology line the same as a SIP trunk?
It works like one, but it is sold as a complete service. Each Wise Cloud Technology line is one simultaneous call, at $29.95 ex GST per unlimited line per month with calls included, and it comes with the hosted 3CX phone system, apps for every staff member, installation on site and support. You are not buying a bare trunk to connect to a system you manage yourself.
How many SIP channels or lines does my business need?
One line carries one call at a time, so you need enough lines for the most calls you make and take at once. A common rule of thumb is about one line for every 3–4 staff who use the phone, though call-heavy teams need more.
Not sure what you need?
Tell us how many calls you take at once and we’ll tell you how many lines you need — honestly.
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