KPN owns the ISRA point (telephone connection) in your house. To report an error on your ISRA point you need to contact KPN through 0900-0244 (€0.10 p/m).
The ISRA point is the point where KPN has assembled calbing. This is usually in the fuse box in your home. An ISRA point might look like in the pictures below. What type ISRA point you have installed depends on when your home was constructed. Older houses have less modern cabling.
The part with the thick KPN cable, you can't touch. Everything that's wired with thin cable you should check yourself and repair, if necessary.
For the analysis of your telephone line, you need a multi meter as the telephone network operates with a low voltage of 48volt.