Matthew Panzarino at The Next Web imagines a service called “VoiceTime” for iPhones:
Imagine firing up the Phone.app from your device’s springboard and making a call. If your phone detects that the other device is running iOS, then it automatically switches seamlessly to an HD audio call using your data connection and not your minutes. Lets call it VoiceTime.
Wonder what this would sound like, but don’t want to wait? Put on your iPhone headset (you know, the headphones that came with your iPhone) and give someone a FaceTime call. Once it connects, the audio you hear is much, much higher quality than what you hear on a normal phone call. If I absolutely have to have a conversation with someone over the phone, this is my preferred method. Looking into the camera is optional. In fact, you can exit to your home screen at any time while on a FaceTime call. At this point it essentially becomes a VoiceTime call.
The caveat is that this feature will probably require wi-fi. At first, anyway.