Blog Carriers and SIP
What is a SIP trunk? A plain-English guide for business owners
“SIP trunk” sounds like something only an engineer needs to understand. It is not. A SIP trunk is simply a business phone line that arrives over the internet instead of a copper wire. This guide explains what you get, what you still need, and how to buy one without being talked into things you do not need.
The one-sentence answer
A SIP trunk is a connection from a phone provider that carries your business calls, and your phone numbers, to your phone system over the internet.
SIP is the language phone systems use to set up calls over the internet: who is calling whom, and whether the call was answered. Trunk is an old telephone word for a line that carries many calls between two exchanges, rather than one phone’s line. Put them together and you have a single connection that can carry many calls at once.
What you get from the provider
- Phone numbers, sometimes called DIDs (direct inward dialling numbers). New ones, or the ones you already have, moved across.
- Channels: how many calls can run at the same time. Two channels means two simultaneous conversations.
- Call rates: per minute, or a bundle of minutes, for calls to landlines, mobiles and abroad.
- Connection details: a server address, and either a username and password or an agreement to approve your phone system’s address.
What you still need: a phone system
A SIP trunk carries calls, but it does not decide what happens to them. It does not know that 1001 is John, that callers should hear a menu, or that the phone should ring someone’s mobile after hours. That is the job of a phone system, sometimes called a PBX. It can be software on a server in your office, or a cloud service.
Think of the trunk as the road and the phone system as the building it leads to, with a reception desk that knows where everyone sits.
SIP trunk or all-in-one hosted phone service?
| SIP trunk + phone system | All-in-one hosted service | |
|---|---|---|
| Numbers and call rates | From a provider you choose, often a local one | From the vendor, bundled |
| Changing phone system later | Numbers and line stay put | Numbers usually have to be moved |
| Local licensing | Your provider is licensed where you are | Depends on where the vendor holds licences |
| Setup | Two accounts to connect | One account |
Words you will meet
- Registration
- Your phone system signs in to the provider with a username and password.
- IP authentication
- No password: the provider approves your phone system’s internet address instead.
- Codec
- How voice is turned into data. Most providers use G.711, which you will see as PCMA or PCMU.
- E.164
- The international way of writing a number: a plus sign, the country code, then the number, as in
+1 555 000 0000. - Porting
- Moving an existing number from one provider to another.
How to buy a SIP trunk
Ask each provider these questions, and get the answers in writing:
- Are you licensed to provide phone service in my country?
- Can I keep my existing numbers, and how long does moving them take?
- How many channels do I get, and what does another one cost?
- What are the rates for local, mobile and international calls?
- Do you recognise my phone system by username and password, or by approving its address?
- How are emergency calls routed, and do my numbers need a registered address?
- Which numbers may I show as caller ID?
Start with the phone company you already use: many offer business SIP trunks even if they do not advertise them loudly. Size your channels to your busiest moment, not your average day.
Connecting a SIP trunk to Vocivo
Vocivo is the phone system half. It connects to your provider directly, so there is no extra box to buy. Its Connect your phone line wizard knows 223 providers in 50 countries:
- Pick your country and your provider.
- Paste or upload the provider’s welcome email. Vocivo fills in the details, or you answer four plain questions.
- Press Connect. Vocivo sets up its side, writes the email your provider needs if they approve addresses, and gives you test-call buttons.
For the technical details your provider may ask about, such as number formats, codecs and caller ID, read Bring your own SIP trunk and How it works.