Our boss was freaking out over people sometimes doing some private calls during work hours and at a certain point absolutely forbade it. So yeah, people would just end the call at 17:00 sharp and switch off the work phone. It took one week before that rule was rescinded.
This reminds me of a work-to-rule or a "White Strike." It turns out that every company, even those that supposedly operate off of "unskilled" labor, utterly rely on employees making a ton of judgment calls and often working outside their job description. When employees start working to the letter of their job description, the whole operation quickly grinds to a halt.
Sure. It means they can ask you to do other things that aren't explicitly written in the original job description. But every time they tell you to do something beyond it, you just start doing THAT exactly to the letter of the request.