18.5. Number plan

When setting up an office phone system you need to consider extension numbers. These are short numbers, typically 2, 3, or 4 digits used to call other telephones on the same system. You can use these numbers to call other extensions and, importantly, use them in configuration of hunt groups and so on, making the config easier to understand.

In addition to an extension number you would normally set a ddi (Direct Dial In) full phone number for each telephone. This is not a requirement and you can just use internal numbers for phones if you prefer.

It is a good idea to make a clear plan for how you will allocate the internal numbers, especially if you have a corresponding block of real phone numbers. Consider which are nice numbers you may want to publish for some reason, and which could be obvious mis-dials. Maybe group extensions by department, etc. Always assume you will need more numbers later so make a plan that allows for expansion.

Tip

It is a good idea to get a block of telephone numbers from your carrier, and use extension numbers that are the last 2 or 3 digits from that block. That means everyone knows the full phone number for any extension. Most carriers can provide a block of numbers. You can then configure these as the DDI (Direct Dial In) numbers for each telephone.

Note

Extension numbers can be any length you like. They should be kept short so they are not confused with local telephone numbers and are easy to dial. There is no need to dial 9 for an outside line as VoIP phones send the whole number when you dial. The only special cases are emergency numbers (112 and 999 by default) which cannot be used as extension numbers.

Note

DDI (Direct Dial In) telephone numbers must be entered in the full international format. This is a plus (+) then the country code, and area code, and number. e.g. +441234567890 which is country code 44 for UK, area code 1234 and local number 567890. In the UK you would dial this as 01234567890. Note that UK numbers have no 0 in front of the area code when quoted in international format.