Michelle,
No, I don't know what might be causing that. It could possibly be a software glitch. It could possibly be an intermittent hardware problem.
Is it always the same caller that it happens with? If so, THEIR phone could being put on mute. Ending the call for you to pull the battery might force the caller at the other end ot clear up their mute condition. If it happens with different callers, then forget that possibility.
The next time it happened, I'd be tempted to put it on speakerphone and see if no sound comes out. Since that uses a different speaker, that might tell if the earpiece speaker had a bad connection or was disabled in software.
These suggestions are groping in the dark, but worth doing anyway:
1. Make sure you aren't having memory problems by keeping an eye on your File Free in Options > Status. If you are having memory shortages that cause this problem, pulling the battery would free up memory but appear to fix the dead earpiece.
2. Make sure your problem isn't being caused by a very low battery condition. Keep your phone charged as close to full as possible. If the problem occurs again, look at your battery charge. If your battery doesn't seem to hold charge well, consider replacing the battery. This is a long shot, but still a remote possibility.
3. If you just can't pinpoint it to a hardware problem, consider backing up your phone (especially Address Book), wiping the phone completely, and reinstalling the OS and the Address Book. Hold off on replacing any themes or apps in case one of them might be to blame. Test the newly loaded phone for a long time to see if the problem returns. If you need help taking this drastic measure, come back for more help.
If you phone is still under a warranty, I'd be careful not to let the warranty expire in case you simply have to take it back. If you do take it back, demand a new, not refurbished phone.
Please keep me informed on what happens.
Kelly