A while back I posted that sometimes I could not make calls with my Droid X, although I could receive them, sort of. Even though the phone rang, and I answered it, there was no communication.
Last week I discovered a similar, related problem. I was unable to receive calls. I tried to call my cell from my house phone. I heard the ringing tone in the phone I was using to call, but my phone acted like nothing was going on. I tried this several times, and then rebooted. That solved the problem. Maybe the problem with the randon reboots some people are experiencing is that the phone knows it isn't working right, so it reboots itself. That's why they call them Smartphones. (Only kidding. of course.)