You can try 1234, or 0000. Those are usually the default codes. If neither of those codes grant you access, a code was set up at some point in time, and T-Mobile does not have access to it. After several failed attempts, the SIM card will lock, and the phone will ask you for a PUK code. At that point, we will be able to provide you with this code, and you will be able to create a new SIM pin.
I think your new mobile is locked with another network ...If it so means you should find which network is locked with your mobile and then unlock it .To unlock your mobile you can get the code from an online vendor like WickedUnlock.com ..