My wife and I both have Blackbery curves. Every so often the audio will go out and when you either place or receive a call you cannot hear on the phone, but the person you are talking to can hear you. After you reset the phone by taking out the battery and replacing it things are fine again.

What would cause this?

Although you are on Verizon, this issue is common on a lot of carriers. Att has this issue also and i find it very nerve racking. Rim states it's a cell path availability issue and there is no resolution. One way transmission. I usually hang up and redial if I
see connected on the display.

Followup, the problem seems to be getting worse. Most carriers are overloaded. Would be nice if this was fixed.