NEAXMAN: When you first open the back of your phone, look at the circuit board. See two black edge connectors? There is a flex circuit that plugs into those black edge connectors. This same flex circuit wraps around to the other side of the circuit board (where the buttons are). This is called a "Flex Circuit" and it is the defective part in your phone. There is also a rubber "membrane" that goes between the actual buttons and the flex circuit. The rubber membrane does not cause this problem.
For what it's worth, I've tried cleaning the mylar flex circuits and I've searched them with a magnifying lens trying to find out what the problem is with no luck. There is a short somewhere in that circuit - but I've never found the problem in 4 of these that I've replaced - and cleaning it did not good either. Please do try to find out what is wrong with it if you can. If you are successful in repairing one of these flex circuits, please tell us what you found.
You said your phone was refurbished. I don't think this part would be replaced as part of refurbishing unless it was acting up during the time it was in for refurbish.
I looked at the sources I use for phone parts - and none of them have this part anymore.
For Partner Plus, Partner II, and Partner ACS Phone systems, I believe the most common phone is the "MLS-18D" or the "18D" - and many "wear" parts are available for them (including the flex circuits). I know for sure that the flex circuit is available now (February 2015) for the Partner MLS-18 and MLS-18d phones. If you have to replace your phone, consider going with one of those - for ease of service. With the MLS-18d phone, if you want wall mount, you just take the back cover off and flip it around, then re-attach it.
Here's some pictures of these flex circuits:
Link