Transfering phone numbers?!

[Master_X]

I currently have a Nokia N72 with t-mobile, and at the end of my contract, im looking to swap networks and go for an iphone BUT i want to keep my number.

My question is should i get the PAC code from t-mobile first then cut my contract then go to O2 and set it up though them when i buy or should i buy the iphone activate it then sort out the number transfer after?

Any help would be much apreciated?

 

[Master_X]

Just an update: I got the PAC code off my previous provider and they are going to send it by post (takes up to 5 days) then i can just walk into the O2 shop and they can change fill in the details there and then but it takes 24 hours for it to complete/authenticate (or whatever).