What Is a Dial Prefix on a PBX?

One word, several meanings

The word "prefix" is used for several different things, which is the source of much of the confusion:

• Area code — the digits at the start of a number: 02 for Jerusalem, 03 for the Gush Dan area, 05X for mobile, 07X for a nationwide number.
• Country code — 972 for Israel, 1 for the United States, 44 for the United Kingdom.
• International exit code — the digits you dial before the country code, such as 00.
• PBX prefix — digits that the PBX recognizes at the start of a number, and according to which it decides how to send the call out.

This article deals mainly with the last meaning, but it's worth knowing all of them — because they meet. When someone in the office dials 00 followed by a number, the PBX needs to recognize that this is an international call and decide what to do with it.

The old-fashioned 9

Anyone who worked in an office twenty years ago remembers: to get "outside" you pressed 9, waited for a tone, and then dialed the number. The reason was simple. A physical PBX had two worlds — the extensions inside and the lines outside — and the digit 9 told the PBX: grab me an outside line.

On a cloud PBX this separation has almost disappeared. The PBX can tell an extension (three digits, 201) from an external number (ten digits, 03...) by length and structure. So in most cases you dial a regular number, and it simply goes out. There is no 9, and no additional tone.

What remains of the 9 is mostly habit. A longtime employee who keeps pressing it does no great harm, but it's worth explaining to the team during the transition that it is no longer needed.

What to do with prefixes on a cloud PBX

Even if you don't need a prefix to get out, prefixes are still useful. With a few digits, they let an employee choose how this call goes out:

• A different caller ID. A prefix that makes the call go out with a different number of the organization — for example, with the donations line instead of the main number. More on this at Caller ID.
• A specific outgoing route. A call that goes out through a particular provider or plan — for example, an international calling plan.
• Destination restrictions. Rules that block calls beginning with certain digits — international, premium-rate content numbers (1-900) — or allow them only from some extensions.
• A hidden number. In many networks and systems there is a code you dial before the number to hide your caller ID on a single call.

Alongside prefixes there are two related tools: speed dial, in which a short code replaces a long number, and routing codes, in which a code triggers an action on the PBX.

How to dial correctly to and from abroad

1. Exit code — 00, or an international carrier code.
2. Country code — for example, 1 for the United States.
3. The local number — without the leading zero, if there is one.

And in the opposite direction: someone calling from abroad to the Israeli number 03-5551234 dials their own country's exit code, then 972, and then 3-5551234 — without the zero. In the international format, which appears on business cards and websites, it is written ‎+972-3-5551234. The + sign replaces the exit code, and a mobile phone translates it on its own.

A common mistake is to keep the zero: ‎+972-03... won't work. Another mistake is to publish a number on a website without a country code, when part of your audience is abroad.

Destination restrictions: why they matter

Calls abroad and to premium numbers are where phone bills balloon. Sometimes it's an employee's mistake, and sometimes it's something more serious: a break-in to an extension and thousands of calls placed to expensive destinations. This phenomenon is called toll fraud.

In many systems, one of the basic protections is a rule that restricts calls by prefix: international only for certain extensions, and premium content numbers for no one. It doesn't replace strong passwords, but it is an additional layer.

In a yeshiva or an institution, restriction takes on added meaning: a public station that students use needs clear limits. More on this at Calling Card and Student Phone.

Examples from the field

A nonprofit with two lines. A coordinator at a nonprofit calls both donors and suppliers. To donors she wants to call out with the donations line number, so they call back there. To suppliers — with the main number. A short prefix before the number determines which number each call goes out with, without switching extensions and without entering settings.

An office with employees who call abroad. Two employees deal with suppliers abroad, and the rest of the office doesn't need to dial there at all. A prefix-based rule allows international calls only from their extensions, and all the others are blocked.

A call center that dials a lot. A team that makes dozens of outgoing calls a day wants to dial quickly, without thinking about prefixes. There the goal is the opposite: for everything to work without any prefix, and for the outgoing number to be set by the extension. And for callbacks to regular contacts — speed dial.

Common mistakes

• Keeping the habits of an old PBX. Employees who keep pressing 9 before every number may discover that the call doesn't go out, or goes out somewhere else.
• Prefixes nobody remembers. A prefix that isn't explained to the team is a prefix that isn't used. A small sign next to the phone or a short email to employees solves that.
• A prefix that collides with a real number. A prefix that starts like an emergency number or like an area code can cause a wrong dial.
• Forgetting the international format. A number saved with ‎+972 and then typed manually, without the plus or with the zero, won't always dial correctly. It's worth checking once how such a number is dialed from the extension.

Examples of prefix rules

The best way to understand prefixes is to see a few written rules. The codes in the table are for illustration only — on every PBX you choose them in the settings — but the types of rules are the ones that recur in almost every organization:

Alongside the "what you dial" rules there is a second family of rules, which look at the beginning of the number that was dialed and not at a code added before it. These are the restriction rules:

How the PBX reads the digits

When you dial a number on an extension, the PBX doesn't wait for you to tell it what it is. It looks at the digits and decides on its own, in a fixed order:

1. Is it an internal extension? Three or four digits that match an existing extension — the call stays inside, without going out to the network.
2. Is there a prefix that was defined? If the number starts with a known code, the PBX strips off the code, remembers what it asked for (a different caller ID, a different route), and continues with the remaining digits.
3. What type of destination is it? Based on the beginning of the remaining number — mobile, landline, international, special number.
4. Is this extension authorized for this destination? This is where the restriction rules come in.
5. Normalization and sending out. The number is converted to a uniform form — for example, a zero at the start of an Israeli number becomes the country code — and it goes out with the caller ID that was set.

Two details from this process explain most of the problems. The first: the longer match wins. If there is a rule for 00 (international) and also a rule for 001 (United States), a number that starts with 001 will hit the second rule. The second: the prefix is stripped before the number goes out. If the code wasn't defined, it isn't stripped, and the telephony provider receives a number with extra digits at the start — and returns "invalid number."

This is also the answer to why a prefix can't start like a real number. A code that starts with 05 would collide with every mobile number; a code that starts with 1 might collide with emergency numbers. That is why internal prefixes almost always start with a star or a pound sign — characters that no phone number starts with.

A full example: a real estate office with three departments

A real estate office in Bnei Brak: a sales department, a rentals department, and management. Three published numbers — one for each department — and twelve extensions. This is how its prefixes were built:

• A default per extension. Every extension in sales goes out automatically with the sales number, and every extension in rentals with the rentals number. No prefix. Most calls end here.
• One prefix for switching. A sales agent who is handling a rentals customer as an exception dials a short code before the number, and the call goes out with the rentals number — so the customer calls back to the right department.
• International for two extensions. Only the manager and the agent who works with investors abroad can dial 00. The other extensions hear a short message: "The destination is not available from this extension."
• 1-900 is blocked for everyone, including the manager. A real estate office has no reason to dial there.

Before the office started working, a quarter-hour test was done: from a sales extension they dialed the manager's mobile and checked which number was displayed; then again with the prefix; then they tried 00 from an unauthorized extension and saw it was blocked; and then from an authorized one and saw it went out. Four calls, and all the rules were verified.

What we learn from this: the first rule — a correct default for each extension — saves ninety percent of the prefixes. Prefixes remain for the exceptional cases, so you need only a few, and they're easy to remember.

What to do when it doesn't work

The last row is the most common mistake with restrictions: a rule that is too broad. "Block everything that starts with 1" also blocks toll-free and service numbers. Block what you need to, and no more.