You guys forgot to tell the most important thing: Which authority to
turn to. Have been in this situation recently myself and found out that
such things are nowadays handled by the "Bundesnetzagentur":
http://www.bundesnetzagentur.de

Write a mail to a woman called Ute Booten (adress is
)
and explain your situation. She will help you quickly and efficiently
(which otherwise is not what german authorities are known for, but
she's the exception that proves the rule).