Has anyone run into this problem?:

I am on the phone with one caller and a second call comes in. I answer the incoming call which puts the 1st call on hold. When I end the second call and go back to the original caller, the screen switches from 'on hold' to active which suggest that we can resume our conversation. Instead the caller can hear me, but I cannot hear the caller--as if mute has been turned on in the wrong direction.

Any ideas how to correct this...do I need a replacement sim (don't know why that would matter)? Do I need a replacement phone?

BTW, this is a Cingular-branded Treo 680 that was unlocked by Cingular.

Whatever recommendations can be provided will be greatly appreciated!