What can be done to keep workers from playing with their phones when whey should be working?

I was at visiting a senior care facility recently and noticed that many of the employees were constantly holding and fiddling with their phones while the patient call button incessantly beeped. They take care of a person’s needs and then whip out the phone and start fiddling while they are walking or sit in the lounge.
There was someone sitting behind a counter working at a computer. I figured she was managing a spreadsheet of employee services. I was forward, and a bit sneaky, and managed to look at the monitor. She was scrolling FB posts. At one point there were 3 people standing in the hall looking at pictures while call lights were blinking for a couple of rooms.
I get that these are minimum wage jobs and sometimes that means you get minimum interest people. But shouldn’t they do their jobs?
What can be done to prevent employees from goofing off with their phones? Are there technical or policy solutions?
A company policy that prohibits phones will not work. What if there is an emergency?
Social media, FB, Twitter, etc, can be blocked from the company Wifi but that would limit patients, too. And the staff could access via their own data plans.
Signal jamming would work but it would disrupt visitors and patients as well.

How can the distraction of social media be eliminated or reduced so employees concentrate on doing their jobs?
Does your company have a policy on this subject?
Have you seen examples of this distraction in other settings?