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Open the hand set by Removing battery, remove 2 screws under battery, carefully open handset casing by prying open with thin bladed screwdriver, remove pc board by removing 3 retaining screws, on opposite side of pc you will see contacts for handset buttons clean with eraser and reassemble handset.
make sure the phones do not have intercom and is stuck on intercom.....buttons will stick on the keypads on these phones and it's possible that you might have sticking buttons....try each button and make sure they are not sticking down.....also, the key pads develop short circuits because of water vapour in the air or other things like pop spilled on them and so on....even if the buttons themselves are not sticking, they might still be shorting out because of a contaminated key pad.....in that case..you'll need to have it opened and cleaned with acetone or PCB board cleaner.....even alchohol will work but not as good as actual cleaners for boards...good luck
After cracked open the handset, I am pretty sure most of problem is not circuit issue. It's the sticky pad somehow aged. I put a small piece of aluminum foil between the rubber key pad and inner plastic conductive pad, and fixed it. No glue needed so far.
To open phone remove the 2 crews at the bottom, then pry open using a screw driver to unlock the internal fasteners. then remove 3 internal screws. Clean-up the PCB key and the back of the keypad with Acetone. This works perfectly. Saved me $150 on a new set.
Something such as coffee or soda may have been
spilled in the phone causing a loss of connectivity between the keypad and the
contacts. The keypad is a rubber membrane with soft round nubs on the bottom
that press against gold circuit board contacts when depressed. It would have to
be disassembled and clean with q-tips and rubbing alcohol or similar solution. This is for a professional or handy
person to perform only
The weakness with the handsets was that women using hand lotion or hair gel can get it on the front of the keypad. It then works it's way to the back side of the rubber keypad. The oil gets on the powder carbon on the keypad. The Carbon shorts the copper on the phone to activate the #1 or #2, etc. When the oil mixes with the carbon the key no longer works. I replaced many keypads until Sony Supplies ran out.... A cheap home fix would be to open the handset and clean the #1 key and the circuit board. You can try the keys but if they don't have any carbon on them you can buy liquid carbon at Electronic Stores OR just paper punch a circle of alumimum foil. Glue the foil on the key with thin glue. Works....