This problem may occurred on the device when it suffered wet or water damaged.
The solution below shows the entire keypad matrix connection and line paths and certain components that also needs to check for further troubleshooting or repair.
Nokia 6700s slide full keypad solution
Some of the numeric keys were connected to a filter coils located at the bottom of the Nokia 6700 PCB board near the battery connector terminal. These particular keys are 7, 8, 9, *, 0 and #, so if one of these keys have the problem, you may also check those certain filters coils being connected to it.
For further troubleshooting and seems that all of the line paths connections and filter coils were both okay but still the problem exists. You may then proceed to check and or replace the Keypad Filter IC located or mounted near the LCD flex pin connector at the right side and top of the PCB board.
Nokia 6700 slide keypad EMI-ESD filter IC
If one of these said filter chips breakdown or being damaged it will result to keypad failure problem issues.
Replacing the filter IC's may only possibly fix and repair nokia 6700 slider keypad problem issues.