We have a Samsung x495 from T-Mobile and were planning on getting an Australian and/ or NZ SIM card to use on upcoming trip after getting the "unlock" code from T-Moble. I called this morning to make sure I could make LD calls to the states using that phone (sometimes that's a separate "unlock") and found out that this particular phone's frequencies (800, 1800 and 1900) do not match up with the 850 in use in both OZ and NZ- SO my "best-laid" plans would have blown up in my face.
Thanks to T-Mobile for letting me know in advance and deflecting frustration! ^