Unfortunately, this has plagued the AT&T iPhone since the original model came out. It's most likely a dead zone causing the issue (I have the sameissue from time to time). You can also get a Micro Cell from AT&T, it's basically just an access point that allows you to pick up a cellular signal at home or work (where ever you set it up).
To eliminate any hardware issues, make sure you're on the latest build of the OS (4.3.3 for the AT&T iPhone 4) - I noticed you're still on 4.3.2. If you don't have a case, I suggest the Apple bumper or any case that covers the metal bezel on your phone, that will help as well.
Also, you can "reboot" your phone. To do this, hold down the metal sleep button on the upper-right corner of your phone until the "slide to power off" slider is displayed, then power off the phone. Wait a few minutes and turn it back on.
Hope this helps!