Imagine if your cell provider had an app you could install that tracked when you lacked cell service but had a stable wifi connection, and automatically shunted your number from being a traditional phone number to a VOIP phone number on the fly - heck, mid-call if necessary. You'd have the same number for both, and it'd be completely invisible to the outside world - they call you, and even if you're in a bunker a thousand feet below ground, if there's a wifi network down there, you can take calls like you're sitting beside the tower. The major providers wouldn't get so much benefit from this, but for the second-tier providers with limited networks(Freedom Mobile[formerly Wind] being the most obvious in my area) it'd be a godsend. Get cheap service, use towers when they're around, and bounce back to the nearest wifi hotspot if you're out of range or in a dead zone.
The technical issues with this are profound, I'm sure - they always are for this sort of thing. But I think it should be doable, and I'd love to see it.