I know of a dentist who has one to stop patients using their phones. As someone who works in a hospital, I can see his point. People answer their phones in the middle of procedures. Some even make calls during them.
I don't know, but we have several signs and yet I have still needed to ask patients to stop. It probably happens once or twice a month. Not quite as frequent is those who try to video their procedure. While I get irritated, I'd get more bothered by not being able to make or receive calls myself if a jammer was installed. A good compromise is to have patients seated near an MRI Faraday cage - it works just as well and doesn't break laws.
