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Do you have the original, credit-card-size, holder the SIM came attached to? If you do, it may have the SIM's PUK (Personal Unblock Kode). If not, you can call T-Mobile and request your PUK. Once you have the PUK, you can block your SIM by entering the wrong PIN three times. After this you enter your PUK and it unblocks the SIM and you can change the PIN.
P.S. You might just try 0000 or 1234 as PINs in the process.
Just checking to see if you were able to lock your SIM using the suggestion above. Please let us know.