[OldWolf]

Giffgaff has the same deal as T-Mobile but with more minutes.
T-Mobile has unlimited internet, thousands of text-messages, and 200 any network minutes
Giffgaff has unlimited internet, thousands of text-messages, and 500 any network minutes

So, phone T-Mobile and say you want your (PUK or whatever you need to do these things) code so you can change your number over to Giffgaff,
Say that it is because Giffgaff has a better deal for the same money, and that apart from that you have no problems with their service.
They will offer you the 200-300 extra minutes for free whether you top up the 15 quid or not in order to keep you on their network.

Hope this is useful.
Has been tested in the UK only.

[Hardz]

I'm pretty sure this works with any network provider if you tell them you want to move networks they'll offer you some bonus things.