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.