Today, after answering two calls, each time the navigation voice stopped (though the screen still showed my route) and the route guidance disappeared from the display in the instrument cluster. Am I missing something or is it a fault I need to get fixed? 

There’s an option to tick to turn off navigation during calls.

Cant remember which menu its in, maybe in sound.

I think you've misunderstood. I don't want navigation to turn off AFTER I close a phone call which is what it's  doing