Wishlist - VOIP-Equipped Cell Phones

Internet phones are nothing new - they've been in common use for at least a decade. You connect into the traditional phone network via internet, maker calls, have a phone number, and so on. But weirdly enough, that doesn't apply to actual phones. You have to choose which type of service you want. For landlines, this is no big deal, but for a cellphone, which can easily lose its cellular connection, it's a problem. Yes, there's apps for cell phones that let you do the VOIP thing, but they're completely separate from its baseline phone functionality.

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.