Number Portability - a double edged sword
Here in ireland, it used to be that each operator had their own prefix, and you could easily tell which operator a person used by the prefix their number had.
The main reason this is useful is cheaper calls within operators ( voda to voda, or o2 to o2 is cheaper than voda to o2 or o2 to voda ). For this reason many people carry two handsets. Also previously someone could have a pbx, with gateways to the various operators, and decide which one to use based on the number. This doesn't work if the person has changed network.
I personally perfered the previous situation where if you changed provider, then you kept most of your number, but the prefix changed.