I'll add a +1 to Sprint problem.

It was happening to me in my apartment, a LOT. My first call resulted in a tech telling me to "leave my phone by the window because that's what she had to do." That really wasn't a feasible answer because it made no difference.

A few calls later I got an airave and it seemed to fix the problem. I noticed if I forced my phone to roam, on VZW/Alltell towers, texts would be delivered immediately, every time... and I wouldn't miss calls any more. If you know something important is incoming, just force the phone to roam.