A dialling plan for ten buildings
Three digits, ten sites, and the one rule you must not break: never make an extension number carry information that is going to change. The guide we hand our customers before the first meeting.
A dialling plan is the one decision in a telephony migration that cannot be undone. You change carrier, you change handsets, you even change platform. You do not change the three digits six hundred people dial every day and that fifteen years of labels, staff lists and habits have engraved.
The single rule
An extension number must never encode information that is liable to change. Not the department, not the floor, not the job. The site, at a push.
That is the only rule, and almost every plan we take over has broken it. The most common form: the first digit encodes the department — 2 for finance, 3 for purchasing, 4 for production. It looks brilliant on day one. Then finance absorbs management accounting, two people move from purchasing to production, a department disappears, and five years on the plan is lying about half the extensions. By that stage nobody dares fix it: fixing it means changing somebody’s number.
What can safely be encoded
- The site. A building does not move department; it closes or it stays. The first digit — or the first two if you have more than eight sites — can therefore be the site.
- Nothing else. The rest of the range is allocated in order, flat, to the next person.
A four-digit plan for ten sites fits in one sentence: the first digit is the site, the next three are a counter. Extension 3042 is the forty-second extension created on site 3. It has no other meaning, and that is precisely its virtue.
The short numbers that are not extensions
Three families have to be treated separately, because they do not follow people’s lives:
- Functions — reception, on-call, technical services. These are groups, not extensions. They carry a memorable short number (100, 110, 120) pointing at a queue, and the membership of that queue changes without the number moving. It is exactly what people were trying to do by encoding the department in the extension number, done in the right place.
- Places — meeting room, loading bay, workshop. A handset attached to a wall, not to a person. It deserves its own range because it follows nobody through a move.
- Equipment — lift, gate, alarm, card terminal. They have numbers, often analogue, and nobody remembers which. The survey finds them. They go in an isolated, documented range, with the equipment named in plain words.
The ten-building case
Across a multi-site estate, two further questions come up, and they have short answers.
Should it be possible to call another site internally? Yes, always. It is the main reason to unify telephony in the first place. With the site as the first digit, it needs no prefix at all: you dial the four digits and it rings, whether the handset is down the corridor or three hundred kilometres away.
What happens if a site is cut off? The handsets on that site keep calling each other through their Edge. Calls to other sites fail — those went through the cloud — and fall back to the overflow rule you defined, usually the local backup line and the remote site’s external number.
What to bring to the first meeting
- The list of existing extensions, with the person’s or the room’s name. An export from the current system, however unreadable, is enough: we read it.
- The list of external numbers, DDI ranges included, with what they point at today.
- The groups that actually exist, including the ones that are not configured and work because “Sylvie picks up when it rings three times”.
- The equipment plugged into analogue lines. That is the list nobody has, and it is the one that derails migrations.
