Using most prepaid phones in other countries requires the phone's user to swap the phone's existing SIM card with one compatible with the wireless networks in the area. The phone may also need to be unlocked. However, there are exceptions to this requirement for some phones.
Cellular phones purchased in the United States typically operate on a different set of frequencies than the cell phone systems used in other countries. Phones that use GSM-type cellular phone systems can often be enabled for use in other countries by swapping the SIM card in the phone for one compatible with a cellular network in the country, but this is not true for phones that use CDMA networks in the United States. Also, since the phone is using a different network than the original prepaid network after the SIM card is swapped, it is likely subject to separate fees for service that are not included in the original prepaid plan.
There are some prepaid phones sold as "world phones" that may already include a internationally-compatible SIM card or a slot for one. These phones may also have options for service in foreign countries that are provided through the US carrier or its affiliates. Many such phones are sold already unlocked or with unlocking enabled for travel, such as the Global Ready phones sold by Verizon Wireless.