Anything that is not illegal is legal. There is no law that states employers can't enforce rules of discipline in their businesses. There is no law about making rules for some employees but others are exempt. For instance some employees have to punch a time clock, some do not. Some employees have to travel for work, some do not.
The excuse about receiving calls from doctor's is not relevant. Make one exception and you have to accept all excuses. My doctor called, my kid's teacher called, my spouse called, my bookie called, etc. All are important to someone. I am sure that your employer has some form to call in and get a hold of you in case of emergency.
One option is to form a union and get it written into the labor contract. UPS drivers many years ago were successful in getting it written into their contract that the tractor trailers had to have an easy plug to hook a radio up. The driver brought their radios to work and plugged them into the trucks.

