I kind of expect Apple to do it in cooperation with the carriers so that devices like iPads and maybe even iPod Touches could use voices. Add the phone app functionality to non-phone devices or enable it in wifi-only scenarios on existing phones.
I'd see it as something carriers could charge some kind of extra fee for as well as call them cell "minutes" no matter how they are used. This allows them to monetize it. Possible opportunity to further monetize it by offering additional phone numbers or other phone features.
Voice functionality on a non-phone device would require an existing Apple iPhone on your plan -- this covers whatever risk Apple feels from people buying only an iPad mini (even with 4G data) and using it as a phablet without buying an iPhone.
Lets Apple enter the "phablet" market without actually having to create another iPhone device itself.
Creates unique, Apple-only feature of making and taking calls using your iPhone cell number on any enabled i-device. iPhone in another room? Don't get up, just answer the call from the couch on your iPad.
I think it's an interesting idea that has just enough value for everyone (carrier, Apple, consumer) that I'm sort of surprised they haven't implemented it.