Modem Crash?

A couple of times over the last week my N7+ has shown red text overlaid over the launcher that said there has been a "Modem Crash".

Just now during a call the call was dropped and I got the same message. It disappears too quickly to read the whole thing. So I only really get a chance to see "Modem Crash".

What's going on? And is there anything I can do to stop it?

Am on the latest security patch etc.