Hi @Yben, my colleagues sent you an e-mail on the 26th but it’s in Dutch so I’ll be happy to give you some more information. Our technical department has examined the incoming line and we have indication that there might be a ‘bridge tap’ present.
Bridge taps are located in or after the ISRA (your connection point) and are the responsibility of the home owner.
To explain a bridge tap more clearly: early on there were analog telephone lines and often more telephone sets were connected to them. All were usually "knotted" together; all devices on a line so that you could listen / talk on each. In some cases these old lines are still connected to the ISRA (but no longer in use) and can cause interferences. This doesn’t always have to be the case which is why it can work fine for a long period but suddenly start causing trouble (this has to do with the way the internet signal is sent).
Getting rid of the tap itself is quite easy, however you do need to identify where the tap is located, otherwise you'll be cutting wires which needn't be cut.
Haaving said all this, in some cases a bridge tap indication can return a “false positive” where there actually isn’t a bridge tap present but there’s a problem with the incoming line. If you’re not sure you can post a picture of your ISRA with the incoming and outgoing connections clearly visible, then I’ll be happy to assist to see if we can determine what might be causing the issue.