VoIP - Local Extension Dialing
This allows you to use the last 1, 2 or 3 digits of your phone number to call other numbers in your block. e.g. if you have 2 numbers 01344400001 and 01344400002, then you can call each other by using 001 and 002 if you set Centrex to 3. This is also used when you transfer calls between your numbers.
This does not work if you have numbers in different blocks.