This is curious. If someone has been working most of their lives and dealt with a variety of bosses and workplace situations how could they not know the answer to this issue for themselves?
I sympathize with their plight but only up to the point where they refuse to act.
A diplomatic statement posed as a question to the supervisor should right this.
There are many different good ways to handle this but all require a face to face meeting. Not passive aggressive behaviour like turning your phone off.
Sit down with the other party get the rules set about when to call you and that will be that. Do it civilly and with good humour. Leave with a smile a handshake and a Thanks for your time.
End of the problem.