said by shmerl
Why can't they use Jingle for instance? Or just regular SIP?
The average consumer wants this kind of stuff to just work... which means they don't want to have to use a different app or change a lot of settings to initiate calls over Wi-Fi.
My experience with VoIP apps is that they just don't work as well as the native call functionality on Android - some have poor sound quality or volume, some don't turn the screen off via the proximity sensor. These faults might be app or device specific, but either way they are just not acceptable to the general public.
There's also authentication. Wi-Fi calling uses the SIM card, and I don't think apps have enough access to SIM data to make this happen.
911 calls are also something to think about - location data must be provided to the PSAP. I'm not sure how T-Mobile deals with this when on Wi-Fi since I have fortunately never had any emergencies at home.
Basically, seamless Wi-Fi calling that works out of the box is actually rather complicated and needs more access to the phone's hardware than most apps have.
