What Is a SIP Trunk?

What a trunk is, and where the name comes from

The word Trunk in English means the trunk of a tree. In telephony, it describes the "trunk" that connects one PBX to the world: not a single phone, but a wide pipe through which many calls pass. The term is much older than the internet — in the first telephone networks, this is what they called the lines that connected the telephone company's exchanges.

SIP (Session Initiation Protocol) is the protocol with which modern PBXs set up and manage calls over a network — a full explanation is in the article on SIP. A SIP trunk, then, is the PBX's "trunk" that runs over a data connection rather than over dedicated copper wires.

The important difference to understand: an extension is a single phone that connects to the PBX. A trunk is the connection of the PBX itself to the telephone network. Calls dialed to your numbers come in through it, and calls the employees dial out go out through it.

Before the SIP trunk: lines, BRI and PRI

For decades, an office PBX was connected to the telephone company through physical lines. A small office kept a few analog lines, one line per call. A larger office moved to digital ISDN lines: a BRI line carried two simultaneous calls, and a PRI line — in the European standard that is also used in Israel — carried 30 simultaneous calls over a single cable.

The problem with all these methods was the same: the capacity is tied to hardware. Want 31 simultaneous calls? You need another full PRI line, with installation, a card in the PBX and a monthly payment. Moved offices? The lines do not move with you easily.

SIP trunking solved this once business internet became fast and stable enough. Instead of dedicated wires, calls travel as data packets over the same connection used for the rest of the work. During the 2000s this became the standard way to connect phone systems, and many carriers around the world are already shutting down the old digital services.

How a SIP trunk works behind the scenes

1. Registration or identification — the PBX identifies itself to the trunk provider, either with a username and password or by a known, fixed IP address.
2. Incoming call — someone dials your number. The telephone network passes the call to the provider, and the provider sends the PBX a SIP request: "There is a call to this number, from that number."
3. Decision in the PBX — the PBX decides where to send it: an IVR menu, a queue or an extension.
4. The voice — once the call is answered, the voice itself travels in compressed data packets, according to an agreed codec.
5. Outgoing call — an employee dials out, the PBX sends the request through the trunk, and the provider passes it on to the landline or mobile network.

The diagram shows the whole path of an incoming call. The trunk is the link between the "telephone network" and "your number" — the point where the call passes from the outside world into the PBX.

Channels: how many calls can pass through the trunk

A trunk is measured in channels — how many outside calls can be active at the same time. Ten channels means ten simultaneous calls, regardless of how many extensions the organization has. An internal call between two extensions does not need a trunk channel at all, because it never leaves the PBX.

With a SIP trunk, the channel limit is mostly a matter of the agreement with the provider and of the bandwidth of the connection, not of hardware. So capacity can be increased or reduced without a physical installation — a big advantage for an organization with busy seasons.

When you need a trunk, and when you don't

• You have your own PBX at the office — an IP PBX in the network closet needs a trunk to talk to the outside world. Here you choose a trunk provider, set up the connection and maintain it.
• You have a cloud PBX — the connection to the telephone network is already part of the service. You don't see a trunk and you don't set one up.
• A special system — a call center with its own software, a dialer system or an old PBX being phased out. Here you sometimes need a dedicated trunk that connects to the existing system.

The practical question is not "do we need a trunk" but "who is responsible for it." With a PBX at the office — you, or your technician. With a cloud PBX — the provider.

Advantages and Limitations

• Flexible capacity — add channels without any new wiring.
• Numbers regardless of location — a local number from another city, or even an overseas number, can arrive on the same trunk.
• Cost — usually cheaper than old digital lines, because there is no dedicated infrastructure.
• Dependence on the internet — if the connection at the office goes down and the PBX is at the office, the trunk goes down too. You need to plan a backup internet connection and alternate routes.
• Security — a trunk exposed to the network without protection is a well-known target for toll fraud. Strong passwords, a firewall and restricting destinations are a must.

Common mistakes

Counting extensions instead of calls. An organization with 40 employees does not need 40 channels. You need to know how many outside calls take place at peak time.

Forgetting the local network. A good trunk won't help if the router at the office blocks or disrupts the traffic — NAT problems are the common reason for calls with one-way audio.

Leaving the PBX exposed. An old PBX at the office with an open trunk and a default password is a recipe for a surprise phone bill at the end of the month.

Example from the field: an office with its own PBX moves from a PRI line to a trunk

An accounting firm in Bnei Brak, 25 employees, keeps an IP PBX in its network closet that was bought a few years ago. It is connected to the outside world through an old PRI line, which provides 30 simultaneous calls — much more than the office needs — and costs a fixed amount every month. The carrier announced that the old digital service will be phased out, and the office has to decide.

1. Measuring — first of all, the technician pulls the call log for a busy month from the PBX. It turns out that at peak time there are 7 simultaneous outside calls, and in the busiest month of the year — 9. The conclusion: a trunk with 12 channels is enough, with room to spare.
2. Checking the internet — 12 simultaneous calls use about one megabit per second in each direction. The office's fiber connection is easily enough, but the technician sets the router to give priority to call traffic (QoS), so that a file backup at noon does not hurt the audio.
3. Setting up the trunk — the trunk provider supplies identification details. The technician enters in the PBX the provider's server address, a username and password, the list of numbers, and the outbound rules (which caller ID number goes out from each department).
4. Running in parallel — for a week the old line and the new trunk work together: outgoing calls already go through the trunk, incoming calls still arrive through the PRI. This way they test sound quality, caller ID and international calls without risk.
5. Porting — the numbers move to the trunk provider. Once the port is approved, incoming calls arrive through the trunk, and the PRI is canceled.

What the office gained: paying for 12 channels instead of 30, the ability to add channels in tax season with a phone call, and an additional number from another city without a new physical line. What remains its responsibility: the PBX in the closet, the router, and security — because this trunk belongs to its own PBX, not to the provider.

When something doesn't work: symptom, cause, fix

Most trunk problems repeat themselves, and each has a distinct symptom. Whoever recognizes the symptom saves hours of guessing.

Rule of thumb: if the call doesn't connect at all, the problem is in registration or identification. If it connects and then degrades, the problem is in the network between the PBX and the provider. And if everything works except one specific destination, it's a policy, not a fault.

Numbers and orders of magnitude worth knowing

• Bandwidth per channel — a call using the basic codec uses about 80–100 kilobits per second in each direction. Ten full channels are about one megabit per second upstream — the direction that is usually weaker on a business connection.
• What replaced what — a PRI line carried 30 calls over a connection of about two megabits. A 30-channel trunk uses about the same, but only when all the channels are actually in use. In quiet moments it uses almost nothing.
• Registration refresh — a PBX that identifies itself with a username renews its registration with the provider every few minutes. That's why a short internet outage isn't always noticed, and a long outage is detected within minutes.
• Call setup — the exchange of messages at the start of a call takes a fraction of a second. If ringing starts only after three or four seconds, something along the way is delayed — usually a request to a server that isn't answering.
• Channels by organization size — rules of thumb from the field: an office of ten employees usually gets by with 3–4 channels; an organization of fifty with 10–15; a service center is calculated by peak hour, not by the number of agents. The full explanation is in the article on channels.

The diagram illustrates the ratio: a PBX with 40 extensions and four trunk channels handles four outside calls, and the fifth caller hears a busy signal. The trunk is the bottleneck — not the number of phones on the desks.

SIP trunk compared with its neighbors

The last row is the big practical difference between a trunk and a cloud PBX: in both, calls travel over the internet, but with a trunk the brain of the system is at your site, and in the cloud it is at the provider. When the office disconnects, in the first case the numbers disappear from the world, and in the second they keep ringing somewhere else. See also On-site PBX vs. cloud.