Good point it could be corrosion. You don't say how many sets you have, but disconnect half the A leads if the problem goes away connect a few, if not hook the ones you disconnect back up and disconnect the others to see if the problem clears. Take RBF's suggestion first just disconnect the amps at all phones if there is corrosion it will be evident, if it's still wet the amp will be warm to the touch.


Retired phone dude