SIP trunking is a way to connect an existing on-site phone system, called a PBX, to the public phone network over the internet instead of through physical copper phone lines. In plain terms, it keeps the phone system you already bought and just swaps out the old lines feeding it for a faster, cheaper internet connection. SIP stands for Session Initiation Protocol, the standard that sets up and ends each call.
How SIP trunking works
A traditional office runs a bundle of physical phone lines from the carrier into a PBX, the box that routes calls between extensions and out to the world. SIP trunking replaces those physical lines with a virtual "trunk" delivered over your internet connection. Your voice is converted into digital data, sent to your SIP provider, and handed off to the regular phone network so you can call any number, anywhere.
Each trunk is made up of channels. One channel equals one concurrent call. If twelve people can be on the phone at the same time during your busiest hour, you buy roughly twelve channels. You can add or remove channels without anyone running new cable, which is the core advantage over copper lines. If you want the broader background on how voice travels over the internet, see our guide on how VoIP works.
SIP trunking vs hosted VoIP and UCaaS
This is the distinction that confuses most people, so here it is cleanly. SIP trunking keeps your PBX; hosted VoIP replaces it.
- SIP trunking: you still own and manage the PBX on site. The provider only supplies the connection to the phone network. You control the system, you maintain the hardware.
- Hosted VoIP and UCaaS: the provider runs the phone system in the cloud. There is no PBX in your closet. You pay per user and the provider handles upgrades, features, and uptime. Read more in what is UCaaS.
Neither is universally better. The right answer depends on whether you want to keep running your own phone system or hand that job to someone else.
Who SIP trunking is for
SIP trunking makes the most sense for businesses that have already invested in a PBX and do not want to throw it away. Common cases:
- An organization with a recent, capable PBX that still has years of useful life.
- A business with IT staff comfortable managing on-site phone hardware.
- A site with specific compliance, contact-center, or integration needs tied to its current PBX.
- A company with several locations that wants to consolidate calling onto internet trunks while keeping local control.
If you have no PBX, or your PBX is near end of life, replacing it with a hosted system usually costs less effort than keeping it on life support with trunks.
What SIP trunking costs
Pricing is almost always built around channels, the count of simultaneous calls you can handle. You typically pay a small recurring fee per channel, then either a per-minute rate or an unlimited-calling plan on top. Because you keep your existing PBX, there is little or no new equipment to buy, which is part of the appeal. To set expectations against a full cloud system, compare with our business phone system cost breakdown.
Pros and cons
The upsides: lower cost than copper lines, no rip-and-replace of your PBX, easy scaling by adding channels, and a single internet pipe carrying both voice and data.
The trade-offs: you still own and maintain the PBX, so upgrades and fixes are on you. Call quality leans on your internet, so you need solid bandwidth and a clean network. And feature growth is capped by what your PBX can do, where a hosted system simply adds features for you over time.
Frequently asked questions
What is the difference between SIP trunking and VoIP?
VoIP is the broad technology for making calls over the internet. SIP trunking is one way to deliver it, by connecting your existing PBX to the phone network over the internet in place of physical lines. Hosted VoIP instead replaces your PBX with a provider's system in the cloud.
How much does SIP trunking cost?
It is usually priced per channel, where one channel is one concurrent call. You buy enough channels to cover your busiest moment, often a small monthly fee per channel plus per-minute or unlimited calling. Keeping your PBX means little new hardware to buy.
Do I need a PBX to use SIP trunking?
Yes. SIP trunking feeds an existing PBX, on site or virtual. With no PBX and no wish to run one, hosted VoIP or UCaaS is usually simpler because the provider runs the phone system for you.
