So, I decided to open the phone by myself, repair the button and write this instructable.
Step 1: Open KS 20
2. Unscrew all four screws marked red on the picture.
3. Put the sharp knife to the left phone's edge between the silver band and the cover.
Make small split between the silver band and the cover and put there the plastic card avid the deformation of plastic parts. Move the card around the cover and open the cover as it is shown with arrows. The cover could be still fixed by the small hooks in the right lower corner. These should be removed at last, when the upper part of the cover is almost free.
5. Move the white part (antenna?) at the right corner up and in clockwise direction. Remove the screw below this white part.
7. Now we have an access to the turn on button. The button contains two parts: the metallised plastic part at the outer part of the silver band and the board with contacts and small metallic membrane that shorting contacts (Figure 5).
Step 4: Clean-up the Button and Assemble the Phone
9. The button board consists of metallic membrane on the adhesive paper that fixes the membrane above the contacts, and two contacts. Remove the adhesive paper from the contacts board. Do not remove the metallic membrane from the adhesive paper.
10. Clean the contacts on the contact board. I used abrasive paper to clean them. Close the contact board with an adhesive paper and membrane. Put the button inside the silver band and assemble the phone from the parts you have.
11. That's it!