There are two competing systems for cellular phone provisioning. Cell phone provisioning is how subscriber data is stored in the phone so it can gain access to cellular networks. The international standard is GSM. In GSM phones, subscriber data is stored on a small card called a Subscriber Information Module (SIM) that can be switched between phones. Some carriers in North America use a competing technology in which subscriber data must be manually entered into the phone. The two systems use different radio frequencies.
Most United States cellular carriers use a system where subscriber data is manually entered into a cellular phone. Their phones tend to only support frequencies used for digital cellular communications in the U.S. If you use service from one of these companies, you must buy a phone that is provisioned for international cellular frequencies and access to GSM networks. Some U.S. carriers, however, already use GSM as their base technology. For those carriers, your phone is likely already provisioned for use outside of the U.S.. However, you may need to call your carrier to have international service activated.
If you are traveling for an extended period of time, buying a cellular phone from a local carrier in the area you will travel is a good idea. Many carriers in foreign countries offer inexpensive phones with pre-paid SIM cards that can be re-loaded. Using a local cellular phone and number is far cheaper than paying international roaming rates for U.S. carriers. By buying a cell phone abroad, it is likely unlocked and not tied to a particular network. Unlocked cell phones can be used with a SIM card from a U.S.-based carrier upon your return.
Cellular phones provided by all U.S. based companies, including ones that use GSM technology, are locked. A locked cell phone is tied to a particular network and cannot be used on other networks. This means that GSM phones bought from a U.S. service provider cannot be linked to a foreign network with a foreign phone number by simply switching out the SIM card. However, a number of private cell phone stores offer international unlocked cellular phones. These phones can be used anywhere in the world, including the U.S., on any GSM network. If you travel a lot and would a U.S. number and an international number, switching the SIM card in an unlocked GSM phone completely switches the network the phone uses for communication.
Unlocking a Locked U.S. Cell Phone
Some services offer to unlock your U.S. based GSM cell phone. This is generally a violation of your terms of service with your U.S.-based carrier, but often will result in a phone that will work with SIM cards for any local or foreign GSM carrier. By unlocking your cell phone, you void any warranty provided by your carrier and may violate your carrier's contract. If something goes wrong with the unlocking process, you are responsible.