I was thinking about this the other day -- something like a Google Voice would work well for a support phone type of operation. I haven't done support in a long while, but last time I did, we would physically hand off a phone, which made for interesting times.
With either the newly oncall or the previously oncall being able to update the number, that handoff now becomes virtual.
There are a few problems with the phone handoff idea, though. One of the problems is there's no safety net: if the guy who has the phone doesn't pick up for some reason, the alert can't auto-escalate. That might be fixable with something like Google Voice, but I don't think they give you a way to manually escalate an alert if you know you can't handle it in time. In both cases, you probably have to manually roll over the number when someone new comes on-call, instead of having the system do that automatically from an on-call schedule.
We've actually been anxiously waiting to try out Google Voice up here. Unfortunately, they haven't yet extended their coverage to include Canada.
Sorry -- I didn't mean to imply that Google Voice was a competitor, rather to say that a Google Voice-like service (with many tweaks) would work perfectly for this goal.
With either the newly oncall or the previously oncall being able to update the number, that handoff now becomes virtual.