Out in the boonies, you're most likely connected to a GSM transceiver. 2G. EDGE. Wide coverage, punches through walls like the Hulk. AT&T really only puts up 3G towers in population centers, and outside of Colorado, you're likely to get poor reception if anything more dense than a single human is between you and the tower you're on. Why? Signal frequency. Colorado just got switched over to a lower frequency to prevent the ridiculous signal attenuation from the EVERYTHING that blocks line-of-sight from where people are, to the tops of the towers.
And the last time I dropped a call on AT&T was when I was calling from inside of an aluminum-sided house, on county-owned land (not even in an actual town). I tend to drive hundreds of miles on my days off. The only real coverage deadzones I've found are radio deadzones near military facilities, or the usual "30 miles between gas stations" areas. But then, my chosen phones have removable batteries, and nobody ever calls in to complain about dropped calls with them.
That said, avoid every LG phone made since Spring of 2009.