i'm on O2 and prety much same advice as Janobi, they will charge you, but its not that dear for a decent number.
i think about £30 for a golden number (one thats easy to remember), usually with sets of double numbers repeated.
i have one of these myself and i does make it easier to remember and of course if you are in business thats a good thing!
Mine did not cost me anything to change mine as i had a crime number and was getting texts from a complete muppet threating me, my wife and my kids.
when he threatened my kids is when i involved the police.
He was even stupid enough to use a mobile that was registered to him!
Idiot got done for harresment and threats to a minor.
He got done big time, as the old bill dont take threatening children lightly!