it should fit
the plug is the "Y" plug
the larger of the two sized plugs
the 1st large plug to the right of the terminal board
usually has less wires

they go in this order
A - B TB Y Z when you have the card situated as you can read the part number

also make sure no one replaced the Y plug on the handset with a Z plug
the keys are spaced differently and wont work for each other

other than that my handset always works

the SX series uses the same LX handset with a different plug style (PY)