sabbath observant jews generally aren't on call for tech matters. Doctors are on call, and will answer their phone, but that's because there's a fundamental difference between tech on call and doctor on call. one is directly about saving lives, the other is not.
It's common to see orthodox jewish doctors who are on call, walk out of a synagouge while praying to take a call. Or for emergency medics to have a walkie talkie that they can be buzzed on if they have to go to an emergency call.
We could get into a discussion about the details of what that means and how a society can function when people would die if certain things aren't taken care of (say maintaining the electrical grid), but that's beyond the scope of this forum. At the most simplistic level, I'd equate it to round the world tech support. We don't need people in one place being able to stay up 24/7 to provide 24/7 support, we can have people located in many different parts of the world handing off the baton as planet rotates. So the same would be true for the sabbath observant techie on call. Most likely its possible for someone else to do it. Of course, this is more complicated in Israel (than say in the US) where the person doing the call will more likely be jewish, but again, that's getting into the weeds and beyond the scope of this forum. In the US, you'd probably see orthodox jewish tech coworkers doing sat night/sunday calls but not friday night/saturday calls for weekend calls.