This feature would make life much easier for people with bad indoor Sprint coverage. I see there's a well-regarded thread at https://help.ting.com/entries/20947922-reducing-my-bill-via-free-texting-and-calls-over-wifi , but that's a heck of a kludge relying on Google Voice and 3rd party apps.
I only recently learned that TMobile has had free WiFi calling for a long time.
I'm not asking for free, though. CHARGE US per minute, just let us route the calls over available WiFi, and you can pocket the airtime savings. At least our calls will go through!
Most of the pieces are already available (and pretty much all Ting devices have WiFi), so I would guess it wouldn't take a whole lot more than a souped up Asterisk server and some custom scripting at your end.
Please consider this feature.