What Is an Extension?

Where the name comes from

The English word Extension means "an addition." In the days of the landline phone, a home or business had one line from the phone company. When you wanted another phone in a different room, you "extended" the line — hence the name. Over time the PBX arrived, which can distribute outside lines among many internal devices, and each such device got its own internal number: an extension.

Today the term is broader. An extension is not a wire or a wall jack, but a record in the PBX that represents a person or a workstation: "Extension 201 — Reception," "Extension 305 — Accounting," "Extension 410 — Dormitory supervisor." Someone calling from outside reaches it through an IVR menu or a transfer, and someone inside dials it directly with three digits.

An extension is not a device

This is the point that confuses people the most, and the most important to understand: the extension is the setup in the PBX — name, number, voicemail box, permissions, what happens when no one answers. The device is just the way to answer it. You can replace the phone, move from the office to home, or answer from the mobile — and the extension stays the same extension, with the same number and the same history.

In practice: the same extension can ring on an IP phone in the office, on an app on a mobile and on software on a computer — all at once. Whoever answers first gets the call, and the rest stop ringing.

And the other way around: an employee who leaves doesn't "take" the extension with him. You transfer it to the employee who replaces him, change the name, and change the password — and customers keep dialing the same internal number.

How an extension connects to the PBX

In a cloud PBX, each extension gets a username and password. The device — a phone, software or an adapter — connects to the server over the internet and "registers": it tells the PBX "I'm extension 201, and I'm here." From that moment, every call to extension 201 is sent to that device. The registration renews every few minutes, so the PBX always knows whether the extension is connected.

This process is handled by a protocol called SIP. It is the reason you can connect devices from different manufacturers to the same PBX: as long as the device speaks SIP, it can be an extension.

There are also extensions that don't register over the internet at all — for example a SIM extension, which works on the cellular network, or an old analog phone connected through a SIP adapter. As far as the PBX is concerned, they are all full extensions.

What you set for each extension

• Name — appears on the display of internal phones and in the call history.
• Ring time — how many seconds it rings before the call moves on.
• What happens when no one answers — voicemail box, mobile, another extension or a hunt group.
• Outgoing caller ID — which number the customer sees when dialing from the extension (caller ID).
• Recording — whether its calls are recorded and for how long they are kept.
• Dialing permissions — in many systems you can restrict where dialing is allowed, for example no international calls.

Tip: before setting up, it's worth writing a simple table — employee name, extension number, where they answer, and what happens when they don't answer. Half an hour of planning saves many "why did this reach me?" calls.

Extension, group and queue — what is the difference

An extension is one person or one workstation. But most of the time you don't want the customer to dial a specific person, but a "service department." For that, the PBX has components that bring several extensions together:

• Hunt group — several extensions that ring together or in sequence. Suits a small office: "whoever is free answers."
• Waiting queue — callers wait with music until an agent becomes free, and calls are distributed among the agents by a rule. Suits a call center.
• IVR menu — directs the caller to an extension, a group or a queue according to the key pressed.

The same extension can be a member of several groups and queues at once. The bookkeeper, for example, can receive calls directly on her extension, be in the "Management" group, and serve as backup in the service queue during peak hours.

How to choose extension numbers

There is no requirement, but there are good habits. It is customary to give each department its own "hundred": 1xx for administration, 2xx for sales, 3xx for service. That way a new employee immediately understands that a number starting with 3 is service. In a yeshiva you can give each building or floor its own range.

It's best to avoid numbers that clash with other codes — for example numbers that start with a digit used for speed dial or routing codes. And it's wise to leave room to grow: if you have five salespeople today, leave a range for twenty.

Common mistakes

• Weak password — an extension with a password like 1234 is an open door to international calls at your expense (toll fraud).
• An extension with no fallback destination — a call that rings forever on an empty extension is a customer who left.
• One extension for several people — if three people answer on the same extension, there is no way to know who talked to whom. It's better to have an extension for each person and a hunt group that unites them.
• Extensions of employees who left — leaving an active extension with an old password is an unnecessary risk.

A full example: planning extensions for an eight-person office

Take an accounting office in Bnei Brak: two partners, a secretary, four bookkeepers and a messenger. Before setup, they sit down with one sheet of paper and build the extension table. This is how it looks in the end:

Note a few small decisions that make a big difference. Administration is 100 and not 101, because that is the extension everyone will dial the most, and a round number is easy to remember. The bookkeepers get their own 2xx range, so when a fifth one is added, she will be 205 and not a random number. The messenger has no phone on a desk, so his extension is a SIM only. And every extension has a destination for when no one answers: no call rings into the void.

In the main IVR menu the customer hears: "For accounting, press 1; for administration, press 2; and if you know the extension number, dial it now." The last option lets a regular customer dial 203 directly, without going through the secretary. All of this is set up in one working day, because the table is already ready.

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

Most extension problems are one of four. Before calling support, it's worth going through the table — often the solution is a cable or a button.

Rule of thumb: if one extension doesn't work — the problem is in its device or setup. If all the extensions in the office went down together — the problem is in the internet or the network equipment, and it's best to start with the router.

Numbers and orders of magnitude worth knowing

• Ring time. One ring lasts about four to five seconds. 20 seconds is four rings — enough to reach the phone, not enough to drive a customer away. In a service queue it is customary to shorten it to 15 seconds; for a manager you can extend it to 30.
• Extensions versus lines. In an ordinary office, roughly a fifth to a third of the extensions are on an outside call at the same time. An office with 15 extensions usually gets by with 4–6 channels; a call center — almost a channel for every agent.
• Registration. The device renews its registration with the PBX usually every minute to a few minutes. So when you unplug a phone, the control panel will show it as "not registered" only after a short while — not that same second.
• Devices per extension. A phone, software and a mobile together are three devices on one extension, and that is enough for almost everyone. More than that usually signals that you need two extensions and a hunt group.
• Extension number. Three digits give up to 900 extensions if you start at 100 — more than enough for most institutions. Four digits are kept for networks with several branches, where the first digit identifies the branch.

Extensions and the rest of the system: how it all connects

The extension is the endpoint of almost everything in the PBX, so it's worth seeing the full picture:

• The call route starts at a number, passes through a menu or business hours, and ends at an extension. If an extension isn't set up correctly, the whole carefully built route ends in silence.
• Recording is set per extension, so a call that passed between two extensions can be recorded in one segment and not the other.
• The call history shows for each call which extension answered, even if it passed between three. This way you know who ended up talking to the customer.
• Outgoing caller ID is chosen for each extension separately: the sales department can go out with one number, and management with another.
• Speed dial and extension keys point to extensions. When you change an extension number, remember to update them too.