WiFi Calling is something that the larger carriers, namely AT&T and Verizon, have strayed away from offering. Basically saying that their network is good enough and they don't need to offer WiFi Calling. T-Mobile launched the service a few years ago, and its customers absolutely love it. One of the bigger features, that many don't talk about, is the fact that when you are out of the country, you can hop on WiFi and call people back home without paying for roaming charges. Sprint launched WiFi Calling this past year, and AT&T has been rumored and planning to launch their own WiFi Calling in 2015.
AT&T recently sent a petition to the FCC to enable RTT, which stands for Real Time Text. As the company wants to use that as an alternative to TTY (Text Telephone), The reason is that RTT is newer, and TTY is obsolete. Additionally, TTY doesn't operate over VoIP. Which is essentially what WiFi Calling is. AT&T also said that as more and more carriers are switching over to VoIP and VoLTE services, RTT is going to be used a lot more.
The company's CEO, said last year that they would deploy WiFi Calling, and only use it as compliment. Basically reiterating that their network doesn't need WiFi Calling, but they are going to offer it anyways, and still continue to invest in their network. Although they'll be moving voice over to their LTE network soon anyways, much like Verizon, and T-Mobile have already begun to do.
Additionally, Verizon said that they are looking into WiFi Calling. But are not prepared to state whether or not they will be using it in their network in the near future. Or as an added feature for their users. While T-Mobile and Sprint have made WiFi Calling pretty big features in their plans that they offer to their customers. In fact, one of T-Mobile's Uncarrier moves last year was bringing WiFi Calling to every phone they sell. Including the Nexus smartphones, which didn't have WiFi Calling beforehand. It's a great feature to have, even if the big two, or the duopoly, don't think it's needed.