I wouldn't be surprised if, in iOS8, FaceTime Audio becomes the default calling option, similar to how, when sending a message, the system will check to see if the recipient is available on iMessage and, if they are, elect to send that message over iMessage.
It's something I'd like to see, as to me it seems like the most efficient use of humanity's available data bandwidth. It's a system which would favour the most abundant source of data available in any given moment: if you make a phone call at home, and FaceTime Audio was available, the call would be made over a fixed-line internet connection using your home wifi. The next step down would be 4G mobile networks, then 3G, and then probably 'legacy' voice cell networks.
Not only is it efficient, but it would contribute to the ongoing marginalisation of mobile operators (another good thing in my view). The arbitrary distinctions between data used for SMS, voice calls and, well, data, would be further eroded, nudging us towards a world in which mobile data is a commodity: pay for what you use, use it how you like.