ok i see you are in the UK, so you bought a usa brand t mobile to work on your UK carrier?
what model is your phone? there are different models out there for different parts of the world, and some models, even if they are unlocked, may not work on other carriers.
this may be the issue here since you purchased off the internet.
was this phone new or used?
or are you not in the UK anymore?
still could be you may have a phone that is not catered to work on the carrier you are currently using?