First, reinstalling the OS did not re-lock the BlackBerry. And, you are just seeing the default theme, which you can change.
See How do I resend my service books? - BlackBerryFAQ
and follow the directions to register the device with the network and resend service books.
Give the SIM is recognized in another phone, I'm guessing the SIM is ok. But if you are still unable to get the BlackBerry to connect, talk to T-Mobile. I'm not sure they will help if they know it is not a T-Mobile branded device, but you can make it clear your question is whether the T-Mobile SIM is working properly.
Posted via BlackBerryForums.com Mobile