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.