Try and register it on their website online. You will need to the SIM in the phone and be able to receive a SMS. I think this works abroad too. Go to:
http://www.t-mobile.co.uk/service/your-account/signup/
For calls to USA there is nothing you need to do. You can call USA for only 3p/min via an 07755 access number. T-Mobiles own rates are not as good as that.
Check out:
http://www.yourcallworld.com/Country...px?country=USA