The carrier is half the equation but only half. Start by checking out which carriers are popular with real people in the places you will use it.

Cell phones are radios. And radios, even digital ones, vary in terms of reception and transmission. Check out http://www.phonescoop.com. Once you know what carrier you want in your area, use the phonescoop "phone finder", rank by average user rating. Then ignore any phone with less than 20 or so reviews. Read the reviews and look for the keyword reception. Pick a phone with good reception for the carrier you want.

Typically Sanyo phones are the best on Sprint. Motorola are the best on Verizon. I can't speak to Cingular branded phones. If you're going for a hybrid device Palm branded Treo devices have had very good radios lately.

Anyway my point is, don't ignore the quality of the radio in the handset.