There are two types of numbers, Internal and External.
Internal numbers are basically short extension numbers for each User which any other User can dial to get through quickly. For example Kurt has internal number 3101 and Courtney has internal number 3102. Courtney can call Kurt on 3101 without having to dial his full External number.
Generally, but not always, the internal number is set as the last four digits of the external number. As an administrator you can set the internal number for each user to whatever you want.
External numbers are numbers that you would normally consider as your 'phone number' and publish to your customers. People can dial these from anywhere. We allocate these numbers to your sites; they could be brand new numbers or existing numbers that you've brought from a previous service.