Glossary Core concepts
What is SIP trunking?
Also called sip trunk, sip trunks, session initiation protocol trunking
SIP trunking is a way of connecting a phone system you already own to the public telephone network over an internet connection, rather than over physical copper lines. The "trunk" is the logical pipe that carries calls between your PBX and your carrier, and SIP is the signalling protocol that sets up, changes and ends each call on it.
What it replaces
Before SIP, a business with an on-premises phone system bought physical circuits from the phone company: analogue lines one call at a time, or a PRI carrying 23 simultaneous calls on a single T1. Those circuits were priced per channel whether you used them or not, took weeks to install, and could only be added in fixed increments. A SIP trunk carries the same calls as data, so capacity is a configuration change rather than an engineer visit, and you are billed for what you actually use.
When SIP trunking is the right choice
SIP trunking suits a business that already has a working phone system and does not want to replace it. If the PBX is doing its job, the handsets are fine and the only problem is the cost and inflexibility of the lines feeding it, replacing the lines alone is far cheaper than replacing everything. If you have no phone system at all, or the one you have is at end of life, a hosted phone system is usually the better answer because there is nothing to keep.
What you are billed for
Most carriers price SIP trunking on a per-minute basis for the calls themselves, plus a monthly fee per phone number. IZT charges $0.009 per minute and $1.00 per number per month. Some carriers instead sell channels, a fixed number of simultaneous calls at a flat monthly rate, which is simpler to budget but means paying for peak capacity all month.