Here's my input, in relation to your post. VZW has offered wifi-based calling already via its (pretty crappy) Messages+ app for several weeks now - sometimes the iOS app's calling feature works, and sometimes it doesn't.
When I connect to another VZW customer, the voice quality is OK. When I connect to a non-VZW customer or a landline, the voice quality is subpar - muffled as most of VZW's calls are. I can take the same iPhone, put a ATTWS SIM in it and make the same calls (on and off network) and the voice quality is much better - intelligible, actually. The ****** call quality follows my VZW SIM - even with the same unlocked devices. IMHO, VZW is purposely gimping their handsets, and look to Android Central to see that it's not just me or iOS devices.
When I call my house landline from my VZW 6s, sometimes it sounds as if my wife were standing right next to me, the clarity is just that good. It seems not to matter whether VOLTE is enable or not. I've made calls on ATT and TMO iphones and as you said had subpar voice quality. All of this anecdotal stuff; I'm sure you can search the internet and come up with a treasure trove of factoids.
One thing however, I am looking forward to trying out 9.3 and seeing how wifi calling works.