Welcome to the forums, and I'd like to start by saying how sorry I am to hear that your phone line hasn't been great since the start, and that it's now gone completely.
Thank you for letting me know that you've already tried another phone on the line as this really helps with diagnostics.
I've done some testing on the line and it seems as if we need to arrange a technician to call out and take a look at things for you. I've sent you a private message to get this arranged. Please look to the top right of this page where you'll see a purple envelope icon, click on that and you'll see my message.