PS: I was assuming above that you're seeing the message on the screen of your computer, not on a digital screen on your phone (although I've never heard of such a message appearing on a computer screen).
If the message is actually on a screen on your phone (and, presumably, you're not getting a dial tone) then the first thing to do is to unplug your ADSL filter and try the phone plugged directly into the phone socket. (If the problem disappears, get a new ADSL filter).
If the problem persists, and there's a horizontal line on the plate for your phone socket, unscrew the bottom part and try using the internal test socket:
https://btbusiness.custhelp.com/app/answers/detail/a_id/21507/c/5106/
If the problem still persists, borrow another phone from someone to see if that works. If it does then, obviously, there's a fault with your own phone. If it doesn't then it's time to contact BT (or whoever provides your phone service) to get the line fault fixed.