I just had a look into the phone cable box of our house: What enters the house is only a twisted pair, being connected to nothing else than the phone. No local earthing!
Maybe there's an earth connection in the distributor box of the street, but I never did see local earth connections at households.


Exactly! That's the mechanism, which leads to heavy surges at the phone lines relatively to local earth. Means, Mehdi can experience heavy surges between phone lines and his local earth, which are strong enough to destroy his detector!

Professional phone applications provide heavy surges suppressors where the phone lines enter the circuitry, you know these little gas tubes which allow arcs to jump over its contacts.

Kai