A PBX dial plan is the set of rules that decides what happens when someone picks up a phone and dials. It covers how long extension numbers are, which digits reach an outside line, where emergency calls go and which codes switch on features. A good dial plan is invisible: staff dial without thinking and calls go where they should. A poor one causes misdials, blocked calls and confused new joiners. This guide shows how to design a dial plan for an office, a multi-branch business or a campus.
What a dial plan includes
| Part | What it decides | Example |
|---|---|---|
| Extension numbering | The internal numbers of people, rooms and desks | 2001 to 2099 for sales |
| Group numbers | Numbers for ring groups, queues, IVRs and conference rooms | 6000 for the support ring group |
| Outside line access | How staff reach external numbers | Dial the full 10-digit mobile number, or 0 first |
| Outbound routes | Which trunk carries each kind of external call | Mobiles on SIP, international on a second provider |
| Emergency numbers | How 112 and local emergency numbers are handled | Always allowed, on the most reliable trunk |
| Feature codes | Short codes for pickup, forwarding and voicemail | Codes starting with * |
| Speed dials | Short codes for frequently called outside numbers | 99 for the head office main line |
Step 1: choose the extension length
Pick one length for everyone and keep it. Three digits give 1,000 numbers and suit small offices. Four digits suit most companies of up to a few thousand phones and leave room for branches. Mixing lengths causes problems, because the PBX has to wait to see whether you are finished dialling.
Avoid starting extensions with digits you plan to use for something else, such as 0 for an outside line or 1 for emergency numbers like 112.
Step 2: give each department or branch its own block
Ranges make numbers predictable. A typical four-digit plan might look like this:
| Range | Used for |
|---|---|
| 1000 to 1099 | Reception, operators and management |
| 2000 to 2999 | Head office departments, 100 numbers each |
| 3000 to 3999 | Branch 1 |
| 4000 to 4999 | Branch 2 |
| 6000 to 6999 | Ring groups, queues and IVRs |
| 7000 to 7099 | Conference rooms |
| 8000 to 8999 | Analog phones, fax machines and room phones on gateways |
Branches connected to one system can dial each other’s ranges directly, which makes calls between sites internal and free. Read multi-branch office phone system for linking sites.
Ranges also make rules simpler. In Voxdial IP PBX, internal call restrictions accept a single extension, a range pattern such as 20__ or a wildcard such as 2%, so you can stop the lobby phones calling the whole management block in one rule.
Step 3: decide how staff reach the outside world
There are two common styles:
- Access digit. Staff dial 0 (or 9) before every outside number. It is clear and familiar to anyone used to an old EPABX.
- Direct dialling. The PBX recognises a 10-digit mobile number or a number with an STD code and sends it out without an access digit. It feels like a mobile phone.
Either works if it is consistent across all branches. Choose one and print it on the phone label.
Step 4: build outbound routes
Voxdial IP PBX uses outbound routes to decide which trunk carries each call, based on the digits dialled. Each route has:
- Dial patterns with prefix, prepend and match rules, so a route can strip the access digit or add a code the carrier needs.
- An ordered list of trunks, so if the first trunk is busy or down the next one is tried.
- A route position. Routes are checked from the top, and the first match wins, so put specific routes above general ones.
- Optional extras: a route caller ID, a route password (PIN), call recording and a time group that limits when the route can be used, for example no international calls after office hours.
A route can also be marked as an emergency route or an intra-company route. Trunks have their own dial prefix and number-reshaping rules for providers that need numbers in a particular format.
A typical set of routes for an Indian office:
- Emergency numbers such as 112, always allowed, on the most reliable trunk.
- Internal branch numbers on another PBX, if branches run their own systems.
- Mobile and local numbers on the main SIP trunk.
- STD numbers on the main trunk, with a second trunk as backup.
- International numbers, restricted by class of service and protected by a PIN or time group.
Who may use which route is set with class of service. See control your office phone bill with call restrictions.
Step 5: keep feature codes and speed dials out of the way
Feature codes usually start with * so they never clash with extensions. In Voxdial they can be changed, and a custom code that clashes with an existing one is rejected. Speed dials are short codes for outside numbers; choose codes that do not overlap extension ranges or feature codes. For the full list of everyday codes, read PBX feature codes.
Common dial plan mistakes
- Extensions that start with the same digit as the outside line access code.
- Different extension lengths in different branches.
- A general route placed above a specific one, so the specific route is never used.
- No backup trunk on the main route.
- Emergency numbers blocked by a restrictive class of service.
- No written record of the plan, so every new extension is a guess.
Document the plan
Keep a one-page dial plan with the ranges, routes and codes, and update it whenever a branch or department is added. Share a simple version with staff: how to dial outside, how to reach reception and how to transfer.
Planning a new phone system or merging branches? Talk to BitVoice for a dial plan review.
FAQs
What is a dial plan in a PBX?
It is the set of rules that decides what happens to the digits a user dials: which numbers are extensions, which reach outside lines and through which trunk, which are emergency numbers and which are feature codes.
How many digits should office extensions have?
Three digits suit small offices. Four digits suit most businesses and leave room for branch ranges. Use one length across the whole company.
Why does the order of outbound routes matter?
Routes are checked from the top and the first matching pattern wins. Put specific routes, such as emergency or international, above general ones.
Can different branches use different trunks for outside calls?
Yes. Outbound routes choose trunks by dial pattern, and each route can try several trunks in order, so each site can use its own lines with another as backup.
