I'm not a teacher***
Here's what I've found:
Longman - get cut off get cut off - to suddenly not be able to hear someone that you were speaking to on the telephone
I don't know what happened - we just got cut off.
Longman - line /phone/
There seems to be a fault on the line.
There was a click, then the line went dead (=suddenly stopped working completely)
I'm sorry, it's a bad line and I can't hear you.
Student or Learner