What do you think happens to that same MD when he is on call, driving, and passing through a tunnel or cell dead zone? When a cell tower has a problem (not unusual)? When he's on another life-saving call where even switching to call waiting would kill someone (as long as we're being ridiculous)?
I'll tell you what happens:
First, MD's largely still use pagers for just this reason, so he still gets the page.
Second, he calls in as usual because a doctor on call is not going to even be on iOS8, let alone a minutes-old update. For just this reason.
Third, if somehow your hypothetical doctor wasn't supposed to be on call, accidentally picked up his wife's phone, which had been upgraded minutes earlier, was relying on cell phones rather than pages, *and* missed an emergency call... the on call service would route to the next MD in rotation.
Fourth, even willful unavailability isn't grounds to lose a license. Repeated cases might cause disciplinary action from the hospital.
I'm genuinely curious why you felt the need to make up such a scenario despite having no idea how the medical on-call system works. Did you think people wouldn't notice? Are you a liar in general, and just enjoy making stuff up and hoping to fool people? Do you actually think you're knowledgeable in this area? Sigh. I know, it's the internet. But I like to think there are real people out there.