Read recently that all the carriers are moving to WIFI routing capability very soon
Verizon is lighting up VoLTE but has made no commitment that I've seen to direct VoIP off-network. Same with AT&T.
They probably don't want the customer service problems inherent with having to explain packet loss on a crappy DSL line to customers.
TMo's weak coverage map and significantly smaller number of tower sites, drove their decision to be first to direct WiFi VoIP to extend coverage, many years ago. You had to use specific phones, but they've had it far longer tha anyone else.
VZ and T may or may not play. Neither really needs to.
Sprint is just confused. They don't know what the hell they want to do. Their lack of strategy has been apparent since they purchased Nextel and they haven't really come up with one yet.
We're still headed for three large companies fully integrated with all services. We just haven't finished what Judge Greene started in 1984 yet. Still multiple telecom company deaths coming -- probably the "easy" way via mergers and acquisitions.
40 MHz of low GHz spectrum is expected to go for over $80B with a B at government auction soon. The smaller carriers are debt loaded and can't even play as that number goes higher each time.
VZ's last spectrum grab came with the requirement they be essentially a common carrier to have that spectrum. They bit. It hasn't hurt them.
Its a real estate and spectrum land grab race at this point. Well, has been for quite a while actually.
VZ and T showed their hand a bit at the announcement of the Apple SIM card. They said no. Nothing about making hardware that is otherwise completely carrier neutral, actually carrier neutral, benefits them in any way. They want to play the hardware subsidy game.
When threatened by TMo's offer to buy out contracts, VZ subtly changed their contracts to not pay down on a straight line. They moved the payoff date as far to the end as they thought they could get away with, and nobody reads the fine print.
That move will cost TMo hundreds of millions of dollars with nothing more than the stroke of a pen.
It's a fun game to watch. Chess on a huge scale.