Your problem can be attributed to two options, first if your keypad is a push button, you need to send your phone to technicians.
The second is if your keypad is a contact pad, you need to open the phone and check if the conductive layer on the pad is worn or is dirty. You can clean it with an isopropyl lequid.
Be very careful when you clean or open the inside of the phone.
Boitumelo @ MYRA YOUTH VIS'SION HOUSE, June 2007