You may be confusing two different definitions of roaming. Usually when we use the term roaming in the cell phone industry, it means using your cell phone outside your regular home service area. This could happen if you have a local or regional plan and are using your phone across state lines, or if you're out of your carrier's service area and must use another provider's network.

Analog roaming, on the other hand, is what happens when you use a cell phone with older networks that operate on an analog transmission technology called Advanced Mobile Phone Service (AMPS), which dates back to the first days of cell phone usage. Nowadays, most carriers use digital networks such as CDMA and GSM that have much better audio quality compared to the sketchy audio quality from analog networks. However, digital networks aren't as widespread as analog networks, especially in rural areas. So if you're stuck on a lonely country road, having an AMPS-ready phone could be a life-saver. You can tell a phone is AMPS ready if the label says it uses the AMPS 800 band. CDMA carriers such as Sprint and Verizon are the few carriers that still sell handsets that use AMPS. Be warned, however, that analog roaming can be expensive.