I had this problem once with AT&T. And it was only in my office - of all places! I could go anywhere else in the house and it would work fine.

After speaking to a technical support guy at AT&T he told me about cell phone signal boosters that you can put in your house to boost the signal. These aren't those stupid metal coil things that you stick on your phone. This is an actual antenna that you have installed on your home that will carry a signal to another antenna inside your house that re-distributes the signal. You can find them (real ones) for between $300 and $3000).

The one I bought cost $500 and my signal worries are gone. Do a search on google for "cell phone signal booster". Only other option I think would be to look at another carrier.