Q: No sound on call.

I'm having a problem when I dial a new call and there's no sound in the earpiece at my end.  Similarly, if I receive a call, I can't hear the other person but they can hear me.  I can even hear the keypad tones as I dial.  The call connects, but there's just silence.  When I've called people, they say they can hear me fine, but I can't hear them.

All other sound ion the phone works, and I've tried adjusting the volume level, plugging/unplugging headphones.  I've tried enabling/disabling the mute button too. 

The only workaround so far is to hard reboot the phone (i.e. hold down home and power button until the Apple logo shows).

Any ideas what's causing this?  Is there any way I can fix this without a hard reboot?