I'm hoping I already know the answer to this and I'll know in about 24 hours but just want to be prepared.

I have upgraded with T-Mobile and as of today have a shiny new Samsung Galaxy S3, which came with a Micro Sim. Previously I've always just passed my SIM from phone to phone to keep my number and contacts, but now obviously this isn't possible.

When I come to turn my new phone on (it's charging tonight), does it activate itself and pick up my number and contacts? There's not a new number with it, just a sort of default PIN. I hope it's not too much of a headache to sort, I really want to keep the same number.

You need to normally activate your new sim card by calling the network. Some let you do this online.

In regards to your contacts, you will need to find way to copy them over to your new phone. If you can moving from a android phone, it may be easier to sync them to your google account. These will then appear in your phone book of your s3 once you sign in with your google account.

Oops, ignore!

Thanks for that, I just realised my contacts are stored on Samsung Kies so that should be ok.

Looks like I might be able to activate my new SIM online, we'll have to wait and see!