Jasen,

there's no need to butcher the phone:
make a vibration sensor and set the phone to vibrate, or a light sensor and
detect the display lighting up when it rings.

I don't mind slaughtering the phone if I can save money :-) It's old
and useless as it is now. I could attach the relay directly to the
vibration motor, but in this case I nees an accumulator before the
relay switch. How do I decide the size of the accumulator and of the
relay?

ASS-U-MING an ATX powersupply you'll only need a small relay to switch the
power button wires

Yes, it's a normal ATX.

you may want to look into settings that can be used to make the phone only
ring if called from your phone....

Absolutely yes.

Thanks a lot.

Mauro