ADSL drops when phone used

[#1]

If someone phones or we phone out the internet connection drops and reconnects at a much lower speed. Tried filters etc.

[#2]

Are you on ADSL or the fibre FTTC service?  Has this been going on a long time or is it a new issue?

[#3]

First thing is to check where the fault lies although it is more than likely a line or line card fault
Disconnect the phone from the line and connect the router to the test socket
Phone in using your mobile - does it disconnect