Hi, this is Andrew, one of the co-founders of PagerDuty.
That's an interesting idea. We've actually thought a bit about adding phone-based triggering to PagerDuty (via a 1-800 number + access code). The idea was to make PagerDuty useful to non-IT businesses like plumbers that also have the concept of out-of-hours on-call duty. From your comment, though, it sounds like this kind of feature would be pretty useful even in the IT world.
The value is that when support staff has real phone numbers available to them, they tend to dial historically responsive individuals directly in order to get a problem resolved, thus creating a negative feedback loop -- if you ignore notifications and phone calls, you get called less by support staff in the future.
Having a floating number -- especially if we could get statistics on who answers and who always ignores them, and if the number could "call up the chain" automatically when nobody answers, would be a useful tool to solve this issue.
Of course, the preference is that human staff doesn't need to call anyone, but it still happens.