What Is SIP Trunking?

How SIP Trunking Works

Picture your phone system as a house and the phone company as the city water main. Something has to connect the two. For decades that was copper wire: one physical line per call. SIP trunking swaps the copper for your internet connection.

Here is the path of a call on a SIP trunk:

1. A customer dials your number from a regular phone.
2. The call reaches your SIP provider, the company you rent the trunk from.
3. The provider sends the call over the internet to your PBX, using SIP to set up the connection.
4. Your PBX rings the right phone extension, just as it did with copper lines.
5. Outbound calls go the other direction: PBX, internet, provider, phone network.

The trunk is sized in channels. One channel equals one live call. A trunk with four channels handles four calls at once. Your phone numbers, called DIDs (short for direct inward dialing), sit on top of the trunk. You can have many numbers on a few channels.

Each call uses a slice of your internet bandwidth, so the quality of the trunk depends on the quality of the connection. Most providers tell you to set your router to put voice traffic first.

Example of SIP Trunking

A plumbing company has an on-site PBX and three copper phone lines. The phone company tells them the lines are being retired. Instead of replacing the PBX, they buy a SIP trunk with three channels, port their main number and two direct numbers to the SIP provider, and plug the PBX into the internet. The phones work the same. The monthly bill drops.

Then a hard freeze hits. At 8:05 on Monday morning, five homeowners call about burst pipes within the same minute. Three calls connect. The fourth and fifth callers hear a busy signal. Each of those is at least a $400 job, so that is $800 that rang busy in a single minute, and the business never knew. The trunk was sized for a normal Tuesday, not for the morning that pays for the year.

What People Get Wrong About SIP Trunking

Owners count phone numbers when they should count channels. They see "main line, office line, and the estimator's line" and think they have three lines. On a SIP trunk, those are three numbers. The number of calls that can happen at once is the channel count, and nobody looks at it until callers start hearing busy signals.

The flip side is the same mistake in reverse: paying for ten channels for a four-person company because "lines" sounded important. The provider is happy to sell them.

The fix is to size channels to your busiest hour and set an overflow rule. Look at the most calls happening at the same time in your busiest hour of your worst season, and add one channel for outbound. Then decide where call number five goes when every channel is full. "Busy signal" is the default, and it is the wrong answer. Forward overflow to a cell, an answering service, or something that always picks up, so a full trunk is never a lost customer.

SIP Trunking vs. Hosted PBX vs. VoIP

• SIP trunking is the connection between a phone system you own and the public phone network. You still own and run the PBX.
• A hosted PBX is the whole phone system rented from a provider in the cloud. The connection is built in. You never think about trunks.
• VoIP is the broad category: any voice call carried over the internet. SIP trunking and hosted PBX are both kinds of VoIP.

If you are shopping for a new phone system, you want a hosted PBX or a simpler virtual phone setup. SIP trunking is for businesses keeping a PBX they already have.

Why It Matters

SIP trunking shows up in a small business's life at two moments: when the phone company retires the copper lines, and when a phone vendor's quote says "SIP trunk, 4 channels" on it. In both cases, the real question is not the technology. It is how many calls you can take at once, and what happens to the rest. An AI receptionist changes that math, because it answers many calls at the same time. Forward the main number to it at the provider, before the call ever reaches the trunk, and a full trunk stops costing you customers. For the broader choice between internet and copper, read VoIP vs. traditional phone system.

The Bottom Line

SIP trunking connects a phone system you own to the phone network over the internet, replacing copper lines with channels. You pay for channels, and each one is one call at a time. Count your busiest hour, not your phone numbers, and send overflow calls somewhere that answers instead of to a busy signal.