Core Concepts
SIP Trunking
By Myroslav Orshak · operates licensed carrier infrastructure · Updated September 2026
SIP trunking is the method of connecting a business's own phone system (PBX) to the public telephone network over the internet, replacing traditional copper lines or dedicated carrier circuits.
How it works
A 'trunk' is the shared connection between your PBX and the outside world; SIP (Session Initiation Protocol) is the standard that sets calls up over the internet. Swap the copper lines for a SIP trunk and your existing phone system keeps working as before, with the same phones, extensions, and numbers. The difference is that calls now travel to a trunking provider, which connects them to the regular phone network.
Capacity is measured in channels, where one channel carries one simultaneous call. Unlike physical lines, channels are software: need ten more for the season, click; need them gone, click. Pricing is either metered (fractions of a cent per minute) or flat per channel, and the crossover math between the two is worth ten minutes of your time.
Why it matters
SIP trunking is how businesses with working phone equipment escape copper-line pricing without replacing anything. Line costs typically drop substantially, capacity stops being a construction project, and the same trunk carries calls for offices in different cities. It's also the transition path as carriers retire copper — the PBX stays, the lines underneath it change.
One concurrent call uses roughly 85–100 kbps each way with the standard G.711 codec — so a 10-channel trunk fits comfortably inside 1 Mbps. Rule of thumb: one channel per three to five employees for a normal office.
Related terms
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Compare SIP trunk providers →Frequently asked questions
SIP trunking or a cloud phone system — which do I need?
It depends on whether you own a PBX worth keeping. SIP trunking preserves your existing system and replaces only the lines; a cloud system replaces the whole stack for a monthly per-user fee. Businesses with a modern PBX and a competent administrator often trunk it; businesses without either are usually happier in the cloud.
How many channels do I need?
Count simultaneous calls at your busiest hour — not employees. A 20-person office peaking at five concurrent calls needs five or six channels. Start lean; adding channels takes minutes with any modern provider.
Will my numbers move to a SIP trunk?
Yes — porting rights apply fully. Single numbers move in days; large blocks from a legacy carrier typically take two to four weeks. The provider drives the process; your job is matching the paperwork to the old account exactly and keeping the old service until the port completes.
Does my PBX support SIP?
If it was built in the last 15 years, almost certainly — SIP is the industry standard. Older analog systems connect through an inexpensive gateway device instead of being replaced. Every serious trunking provider publishes configuration guides for the common PBX platforms.