All phones currently are "multi-band" and will work on all systems. If you've had your phone for it's contract period, go to your provider and have it "unlocked". You'll then be able to roam more freely and use simcards of other providers, overseas, etc. If locked, your phone will seek it's own providers systems first, regardless of what else is out there.
A cell tower is only good for about 30 miles radius else a problem called "phase error" creeps in. It's a digital vs. speed of light problem. I ran into this last week in the desert. At some point, no provider will have service due to distance with a digital phone. This is why ham radio works and cell won't...analog vs. digital.