I have the same problem here in Baltimore. From everything that I've read, I'm convinced that AT&T has oversold their network. I've been told that if one is on a call, and another user either initiates a new call, or enters your serving cell while on a call, if that cell is at capacity, it will drop the oldest call, in favor of the latest call(s).
Apple diagnostic software (Genius Bar) will read this a a modem reset, but I'm convinced that AT&T is to blame. The dropped call problem is so bad in my office, (4 bars, -75db signal) that I make all outgoing calls using Skype & our WiFi connection.
It is really such a shame that Apple partnered with such a poor carrier (AT&T). Even though CDMA has its limitations, if Verizon ever gets the iPhone, I'll gladly pay AT&T's ETF to rid myself of their dismal service & coverage.