Very funny. Wireless operators will want to charge for the features that you use on the network too. They've already started planting the seeds for this.
In Switzerland, operator Swisscom has launched some interesting packages last year, called "Infinity". You basically get unlimited phone calls, texts and data - and all of them are really flat (no "maximum 10 GB" or similar rules). The only caveat that applies is a fair use policy that allows you to only use it for personal usage (ie. you can't connect your phone to an office PBX or use it as your only Internet connection in a house with 5 people). But you can tether your life away with it. I have friends who actually do BitTorrent download with that and never had an issue. And you can literally stay on the phone all day - it's flat, period.
The price plans are differentiated by bandwidth. The more Internet speed you want, the more you pay. They come in 0.2, 1, 7, 21 or 100 Mbit/s. I currently have the 21 Mbit/s plan and yes, it is expensive, around $130/month. But if 1 or 7 Mbit/s are enough for you, it starts at around $80/month.
With this plan, I never get a bill that exceeds that amount. I even have some roaming minutes and megabytes included for when I'm abroad. And with such plans, people will not be interested in "FaceTime voice calls", it's just not worth the hassle.
Still, as not all carriers have this, of course something akin to FaceTime voice calls is nice to have. I have experimented with Tango and Viber, as well as Skype. Skype is by far the worst (especially because it doesn't support true push notifications, but just lingers in the background sucking battery). Tango and Viber are quite comparable in voice quality (and they're both better than Skype in my opinion), but Viber has a nicer interface. They both use some sort of enhanced (G722?) voice codec, giving that HD-voice like sound.