Most cellular telephones that operate on the GSM standard and have a SIM chip installed can be easily switched between carriers by installing the specific SIM chip. Some carriers, however, lock the telephones ESN or electronic serial number to their service when you purchase a telephone, precluding you from using other carriers' chips on that phone. To use that phone, you need to unlock the ESN.
Locate the ESN under the the battery or in the phone's information menu. Write down this number. Also locate the phone's provider; many phones are marked in some way, either the first screen on startup or on the phone itself.
Call the customer service of the provider. Tell them that the phone is off contract and you would like to travel overseas with it and need to use another SIM. Request they unlock it.
Follow the instructions provided by the customer service representative. This usually involves punching in a number of codes. The phone's ESN will be unlocked and usable on any carrier