Number 1 issue I see with the Treo 680 is the use of a set of headphones will occassionaly make a small switch inside the phone's jack freeze up. This essentialy makes the phone believe that a headset is still attached even though one had removed the headphones. This hardware "bug" will continue to route incoming calls to the headphone jack as opposed to the earpiece.
This is easily fixed by putting a few drops of WD40 or some other light penetrating oil onto the jack of the headphone and inserting it into the phone several times(NEVER spray WD40 or ANYTHING into the Treo). Sometimes the switch will loosen up immediately, other times it may take repeated applications of WD40.