What is a SIP trunk? It is a virtual phone line: a service from a carrier that connects your business phone system to the public phone network over your internet connection instead of copper wires. One trunk can carry many calls at once, and adding capacity is usually a change on the carrier’s side with no new wiring. SIP trunks have been around for years and remain one of the most common ways a business phone system reaches the outside world.
Key takeaways
- A SIP trunk’s capacity is the number of calls it can carry at the same time, sometimes called channels.
- Most providers charge by the minute, by the channel or as a per-user bundle. Pick the one that fits how your office really makes calls.
- You have the right to keep your phone numbers when you switch. Don’t cancel the old service until the numbers finish moving over.
- Strong passwords, accepting traffic only from your phone system’s internet address, limits on international calling and encryption keep a trunk from running up a surprise bill.
- Every business phone system still has to handle 911 correctly.
What is a SIP trunk, technically?
SIP stands for Session Initiation Protocol, an internet standard first published in 2002 for setting up, managing and ending calls and other communication sessions. A SIP trunk uses that protocol to connect your phone system, called a PBX, to the regular phone network. Staff keep their desk phones, softphone apps and extensions. The trunk carries calls in and out.
A traditional phone line carries one call over one pair of copper wires. A SIP trunk carries many calls over your internet connection, and how many it can handle at once depends on what you buy and how much bandwidth you have.
SIP trunk vs hosted VoIP
With hosted VoIP, the provider runs the phone system in its own cloud and you rent seats. With a SIP trunk, you own or control the phone system, either on a server in your office or in your own cloud instance, and you choose the carrier separately. That gives you more control over features and carrier choice, and it makes the phone system’s security and updates your responsibility, or your IT provider’s.
How we set it up for clients
Our clients buy their SIP trunks directly from the carrier, and we set up Microsoft Teams calling through Operator Connect, a hosted business phone system, or both. You own the carrier relationship, and we handle the phone system and the network it runs on.
- Capacity grows with you. More call capacity is usually a change on the provider’s side.
- Your phone system stays. If your PBX supports SIP, you can switch carriers without replacing phones or retraining staff.
- Numbers follow you. They aren’t tied to a building, so a move or a second office keeps the same numbers.
- Calls keep flowing in an outage. If the office loses power or internet, calls can route to a backup site, cell phones or a softphone app, depending on configuration.
- Voice rides on the same connection you already manage, which simplifies billing and support.
For what sits on top of the trunk, see our post on the most helpful VoIP features for small businesses. If your staff already work in Teams, our Teams setup checklist covers the groundwork.
How SIP trunks are priced
| Model | How it works | Good fit |
|---|---|---|
| Metered | A small fee for the trunk or each phone number, plus a per-minute rate. Usually no hard cap on concurrent calls. | Offices with light or unpredictable call volume |
| Channel-based | A set number of channels, each carrying one call at a time, with local and long distance included. Once every channel is busy, the next caller gets a busy signal. | Steady volume and a fixed budget |
| Bundled per user | Priced by user or seat, with calling, numbers and features included | When the trunk and phone system come from the same vendor |
| Hybrid | A base number of channels with metered overflow | Steady volume with occasional spikes |
Whatever the model, ask about fees for phone numbers, toll-free minutes, international calls, text messaging, e911 and porting, which are often billed separately. Size the plan from your actual busiest hour. Your current phone system’s call reports will show how many calls run at the same time.
Moving your numbers
Under FCC rules, you can keep your phone numbers when you change providers in the same area, and your old carrier can’t refuse to release them even if you owe a balance or termination fee.
- Get a recent bill showing the account number, the exact service address and the authorized contact.
- List every number, including fax lines, alarm lines and numbers that only forward somewhere.
- Submit the port request through the new provider with a signed letter of authorization. A mismatch in name or address is the most common reason ports get rejected.
- Set up and test the trunk with temporary numbers before the cutover date.
- Cut over, then test inbound and outbound calling, caller ID, voicemail and 911.
- Cancel the old service only after the port completes. Canceling first can mean losing the numbers.
The FCC requires simple ports to be completed within one business day, but business ports with many numbers or lines often take longer. Most of the ports we handle finish in under 15 days.
Security and 911
An exposed phone system is an attractive target. The classic attack is toll fraud: someone gets into the PBX or trunk account and places thousands of calls to premium international numbers over a weekend, and you get the bill. Attackers with access can also spoof your numbers in scams.
- Use long, unique passwords for every SIP account and extension, and change any defaults.
- Restrict the trunk to traffic from your phone system’s IP address, and keep the PBX behind a firewall.
- Block international and premium-rate calling unless you need it, and set daily spending limits with the provider.
- Turn on encryption (TLS for call setup and SRTP for audio) where the provider supports it.
- Patch the phone system like any other server.
- Choose a carrier that participates in STIR/SHAKEN, the FCC’s caller ID authentication framework, so outbound calls are signed as legitimate.
On 911, Kari’s Law requires a multi-line phone system installed after February 16, 2020 to let users dial 911 directly without a prefix and to notify someone on site or a central location when a 911 call is made. The FCC’s RAY BAUM’S Act rules also require a dispatchable location, meaning the street address plus details like floor or suite, to go out with the call. If you record calls, treat recordings like any other sensitive file, since they can hold card numbers, health details or legal matters.
Idle lines and a phone system nobody has checked in years are worth a look before the next bill. Book an intro call and we’ll go over yours with you. Our unified communications services and managed IT pages explain how we support it.
FAQ
Frequently asked questions
What is SIP trunking?
SIP trunking is a service from a carrier that connects your business phone system to the public phone network over your internet connection instead of copper lines. One trunk carries many calls at once, and adding capacity is usually a change on the carrier’s side with no new wiring. Staff keep their desk phones, softphone apps and extensions.
How does SIP trunking work?
Your phone system (a PBX, on a server in your office or in your own cloud instance) uses Session Initiation Protocol to set up, manage and end each call. The trunk carries those calls between the PBX and the regular phone network over your internet connection. How many calls it handles at once depends on the channels you buy and your bandwidth.
Sources: IETF RFC 3261: SIP, Session Initiation Protocol, 2002; FCC: Porting, keeping your phone number when you change providers; FCC: Multi-line telephone systems, Kari’s Law and RAY BAUM’S Act 911 requirements; FCC: Dispatchable location for 911 calls; FCC: Combating spoofed robocalls with caller ID authentication (STIR/SHAKEN); Microsoft Learn: PSTN connectivity options for Teams Phone.
