And why are bosses calling? If they are calling to see if you can cover another shift at work, for example, then you will be paid, possibly overtime pay, to cover the shift.
A blanket ban to not allow contacting people outside of work hours seems extreme to me.
And worse yet the example talked about email. The employee can choose to read or act on the email — or not. If the phone is on silent the email will not disrupt anyone...
Being called to work an additional shift is only one possibility.
The boss might also be calling, e.g., to find out how to convert a Microsoft Word file into a PDF... or because he needs you to explain to him how to create a Table of Contents... or because he can't figure out how to turn off the spellcheck, or create a header in a PowerPoint, etc.
I had a boss like that once... Was the most-inept fellow with software you could imagine... And was always very apologetic... "Really, it will take only a minute!"... But, of course, it took much longer than a minute...
Regards,