so.. assuming that the bullet listed in the slide 'Wi-Fi Calling' Image
is NOT actually referring to the 'Continuity' feature announced separately that allows a cell call to be answered on another Mac.. (that some of you may be confusing this with)
then implementing voice over wifi (calling) is actually a very logical move for all next generation cellular networks (& devices).. as the LTE enabled networks & beyond will transition all voice to voice over ip (over whatever transport the carrier supports).. most of today's devices are hybrid data over LTE & voice over GSM/CDMA.. but, will start to transition to both full voice & data over ip over ..[insert future data network here]..
T-Mobile, & a couple of others had basically implemented a discrete voice over ip client & gateways (UMA/GAN) that leveraged a carrier-integrated voice over ip network - as mentioned previously that helped addressed short-comings in their own network coverage, but, also enabled cheap or free calls for anyone that had wifi access..
just to reiterate & at the risk of repetition.. sorry long post.... in the not to distant future all voice will ride whatever underlying data network is available (wifi, cellular, etc..) - the (current) constraint is the degree to which it's been implemented at the carriers infrastructure or client device.. & Apple supporting VoLTE in iOS 8 is great news..
Man, if only you had Mark-up, you could have magnified and circled the "Wi-Fi calling" line so I would not have had to look all over the slide for it.
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Basically, in my thinking, if this was true wifi calling, the cell phone service providers would be very unhappy with Apple doing this. Much use of this would likely cut right into the goal of tabulating minutes/data. If I'm AT&T, Verizon, etc, I'd be VERY UNHAPPY with such a move. So, Apple would run it by these very important partners and then sell it by showing them how it will cleverly still burn just as much minutes/data while cutting demand on their cellular networks.
In the US, I wouldn't be so sure AT&T and Verizon would care. Both are major players in the ISP market and will get your traffic (and send you a bill for it) either way. If your call is on WiFi, then there should be less tower traffic, will delay the need for cell improvements, and may lower capital improvement costs.