Can hear caller only on speakers

[LAFitz]

I have a MyTouch 3g running Android 2.2.1 that worked just fine for everything until a couple of weeks ago when suddenly I was unable to hear the caller (and they were unable to hear me) unless I switch to the speakers from Menu after answering the call. All other audio functions work well: I can play & hear music either with or without a headset; I can hear the caller through the headset; and I can hear calls and notifications. The only problem is the in/out audio on the phone itself. Any suggestions?

Laurie