What is a SIP trunk?

The plain version

Your phone system, the box in the closet or the software in a data centre, handles the calls inside your business: extensions, transfers, voicemail, the front desk ringing the back office. To reach anyone outside, it needs a way onto the public phone network. For decades that was a bundle of copper lines from the phone company, one call per line.

A SIP trunk does the same job over a data connection. SIP is the signalling protocol that sets calls up and tears them down; the internet standard describes it as a protocol “for creating, modifying, and terminating sessions” (RFC 3261, IETF, 2002). A “trunk” is the old telephone word for a shared path that carries many calls. Put them together and you have a shared path for many calls, set up by SIP, running over your internet or a private data link.

What you actually buy is:

• Channels, the number of calls that can be up at the same time
• Phone numbers (DIDs), moved over from your old carrier or issued new
• Calling, local and long distance, usually by the minute or in a bundle
• 911 service tied to your building’s address

When a SIP trunk fits

• You own a phone system already. A Yeastar appliance, an older system with a SIP gateway, or a hosted system that needs an outside connection. The trunk is how it reaches the world.
• You pay for more lines than you use. Copper lines are bought one at a time. Channels can be sized to your busiest hour, and the numbers don’t have to match the channels.
• You have more than one site. Several buildings can share one pool of channels and one set of numbers.
• The copper is going. Where the incumbent is retiring copper, a trunk is one of the ways to keep the phone system working. Our POTS line replacement article covers that.

When it doesn’t fit, or needs care

• The connection is weak. A trunk is only as good as the link it runs on. A rural site with one wired connection and no backup path needs a plan for the day the link drops: a second path, a cellular failover, or a copper line kept for the essentials.
• Life-safety circuits. Elevator phones, fire panel diallers and alarm lines have their own rules. Moving them onto a trunk is possible, but it’s a separate decision with the elevator contractor and the alarm company involved. See the elevator emergency phone.
• Very small offices with nothing to own. If you have five people and no system, a hosted phone system already includes the outside connection. You don’t buy a trunk separately.

What to ask a SIP trunk provider in Canada

These are the questions we’d want answered if we were buying.

1. Are you registered with the CRTC? The CRTC says companies offering local VoIP service in Canada must register with it, as a reseller or, if they own transmission facilities, as a competitive local exchange carrier (CRTC, 9-1-1 obligations of local VoIP service providers). Ask which, and ask to see it.

2. How is 911 handled? The same CRTC page says local VoIP providers must provide Basic or Enhanced 9-1-1 and must tell customers about any limitations in that service, at sign-up, in marketing material and in the terms of service. Ask for the notice in writing. Then ask who registers your building’s address against your numbers, and who checks it after a change.

3. Can we keep our numbers? The CRTC’s consumer guidance says you can keep your number when you change local phone companies, as long as you stay within the same local exchange (CRTC, choosing a local phone service). Ask how long the port takes, and who plans the cutover around it. The losing carrier usually sets the date.

4. Is it tested with our phone system? Yeastar publishes a list of providers it has certified for Canada (Yeastar ITSP partners, Canada). Certification means tested interoperability and a configuration template. It says nothing about the provider’s network, support or price. A provider that isn’t on the list can still work; it just needs configuring by hand and testing.

5. Who answers at 3 a.m.? When a call fails, the fault can sit in the handset, the phone system, your internet link or the carrier. Ask who will look at all four, and what their response time is in writing.

6. What happens when we leave? Ask how your numbers, configuration and records come back to you.

Where we fit

We sell SIP trunks on the Heliatel certification lineage, which is how our trunks appear on Yeastar’s certified list for Canada. We sell them to partners and to businesses keeping a system they already own. Details are on the SIP trunks page.

Both of our service agreements require our own trunks. That isn’t an upsell. When one party is responsible for the phone system, the trunk and the circuits, a fault at 3 a.m. has one owner and nobody to point at. If you’d rather keep your own carrier, we’ll say yes, price the agreement differently, and be honest about the part of the path we can’t see.

Frequently asked questions

Is a SIP trunk the same as VoIP?

VoIP is the broad term for voice over a data network. A SIP trunk is one specific use of it: the link between a phone system and the public network. Your desk phones may also use SIP to talk to the phone system, but that part stays inside the building.

How many channels do we need?

As many as the most calls you have up at once on a busy day, plus a margin. That’s usually far fewer than the number of phones. We size it from your call records where they exist.

Will our internet handle it?

Each call uses a small, steady amount of bandwidth. The bigger questions are whether the link is stable and what happens when it drops. A Telephony Health Check looks at both.

Does 911 still work on a SIP trunk?

It should, and it has to be checked. The address registered against your numbers is what the emergency call centre sees. We test 911 on every install and every year under our agreements.

Can we use a SIP trunk with Microsoft Teams?

Yes, through a phone system or a certified gateway between the two. Ask us for the details for your setup.