Update 2013-04-04

Update 2013-04-04: This way is limited to EDGE/2G; there's a better way.

Perhaps you have a Motorola Razr M. It supports GSM in addition to Verizon's CDMA, so it should work on T-Mobile and ATT except that it's been set to reject SIMs from non-Verizon US-networks. The Australian version of the phone is almost identical, but doesn't have this restricti...