A bit late but....
Essentially, your issue is either that the keypad has some gunk under it, or the contact has become worn on the keypad.
Two options:
1. Cleaning - disassemble the unit and then clean the surfaces of the keypad and related board with 99% alcohol.
2. Pad repair - there are keypad repair kits on the market that work fairly well - search for "rubber keypad repair kit" in any search engine and there will be lots of sites that offer them.
I repaired a cordless phone with a cleaning issue - it's posted on my blog at www.electricalrepairlinks.com if you need to see some pics.
Regards,
Tim
Tim
January 2007