My job is pretty flexible about hours and schedules. You can flex your time later if you have a late meeting. Their rule is you still need to have your work cell on you during any time flexed within the normal 8:30-430. So if I start at 9:30 I still need my phone on me from 8:30 to 930.
One reason for this is random drug testing. I can get a call and have to report within a half hour.
To me this sounds like being on call if I'm expected to answer calls in off time. I'm not paid to be on call.
This is in PA if it matters.