The phone lines all show the letter "X". What does that mean?

If none of the phone lines appear to be working, there are several possibilities as to how to fix them. 

One possibility is that all the phone lines have a big "X" next to them, both on the console and on the phone (as demonstrated in the video below).  

If that's the case, chances are someone pressed the "Block All" button on the phone.  Watch this short video to show how to turn Block All on and off.