Except for the U.S. and a few countries in the Far East, Global System for Mobile Communication or GSM is the international cell phone standard which is used by almost all of the countries in the world. It is possible to carry one GSM cell phone and use it almost anywhere if a U.S. service provider has roaming agreements with foreign service providers and if the phone is world, tri or quad band.
There may be a need to switch the frequency of the phones when one arrives in a foreign country, using the menus that will make it work, although most phones do this automatically now. Roaming applies and callers must call a U.S. number to reach a phone whose SIM card has not been changed. A new SIM card can be purchased so that the phone will have a local telephone number thus all calls to and from the cell phone in the country of purchase will be local calls.