Here's the Nokia 3710 fold keypad flex membrane line connection ways and tracks that can be used to trace a certain keypad keys that stop responding or not working when being pressed. This solution can be helpful to trace if there is one or more particulars keys that does not work before trying to figure out the main PCB board components that holds the keypad connection like the keypad filter IC.
The solution below shows the navigation and alphanumeric keys connection between each other including the volume control buttons like the volume up and the volume down.
Each keys have a designated color code for easy tracing procedure. The outer and inner core where both have different connection with each other. Just refer to the solution above for tracking particular keys that is not working or having a problem. For further troubleshooting nokia 3710 fold keypad problem or if this solution already used or already tried installing a new keypad flex matrix but the problem still exists see the Nokia keypad filter IC jumper ways tracks solution