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
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
There are two keypad ESD filter IC being used on nokia 6700 slide, both numeric and navigation keys like, soft left, soft right, up, down, left, right, select, menu, and S60 keys also connected to this filter IC's.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.

