In older telephone installations there are several telephone sockets in the apartment and some of these are sometimes connected to the first socket which comes from the distributor and such outgoing lines can then act like an antenna.
The ideal is a direct line from the external supply line to the socket where the Internet box is connected, i.e. without any lines being connected that are no longer used.
You can simply disconnect such cables.
Installationen, Netzwerk, Internet, Computertechnik, OS Windows, Apple und Linux.