If there is an obligation to respond within a certain time, and an obligation to avoid certain activities (whatever you imagine that makes you unable to respond) then those are work hours even if nobody calls. It's exactly like a fireighter shift or call center shift where there happen to be no incoming calls, or a night shift at a remote gas station if no customers come during that night.
And if you're getting paged outside of work hours, then there's zero obligation to answer your phone.
There's no middle ground. If the employer says that these aren't working hours, they have zero right to ask what you're doing at these hours, much less put any conditions on it; it's your right to spend that time fishing in a remote lake with no cell phone service or go on a date or whatever and not even explain anything without any reprimand when you arrive at your scheduled start of work time; and if they want you to commit to a shift, well, "a shift" where employer tells you what to do (e.g. do not go fishing to a remote lake and don't sleep for 12 hours with your phone off) is by definition work hours.
It is quite plausible that most of the times most employees will pick up the phone and solve reasonable issues, but that's an extra courtesy from the employee, going beyond what you can demand or expect; but the moment you start to require that, or ask an employee to precommit that they will definitely be monitoring their phone for rapid response, that means you're effectively assigning those hours as work hours.
It's understandable that being on-call can be very light work in many cases (not all - quite a few counterexamples in this discussion), so you can agree on different compensation for them, but those definitely are work hours (they definitely aren't non-work hours, and there is no middle ground) and thus any rules on length of shifts and rest between shifts can and should apply also for on-call hours.