If your provider is AT&T, and if your have an iPhone, which usually uses their relatively snappy 3G network (with the even faster 4G coming sometime), then your can turn off the 3G and talk over much larger areas of the USA on their older and slower but much more wider coverage area EDGE network.

Go to Settings>General>Network>Enable3G and turn that to off. In a few seconds you will see the "3G" in the upper left hand corner of the screen be replaced by a bold leter "E" (for EDGE)

I've used this switch back to the older network once so far when the phone said I otherwise had no service. Also texting will sometimes get through even when voice won't.