Landline busy when ringing in, no dial tone on handsets

This seems to be a common theme, but for the last week we have had no dial tone when we pick up either of the handsets we have - one connected to a wall socket and one plugged in as an extension. If we ring the landline number, it comes up as line busy. We have unplugged everything, and tried plugging a different handset into the wall socket, but it makes no difference. I don't think that I have had a letter with an adaptor, and certainly haven't had a text message, so I don't think we've been switched over yet.