https://www.cnet.com/news/what-is-wifi-calling-tmobile-verizon-att-google-fi-sprint-setup-faq/

And just as carrier technology continues to evolve, so does Wi-Fi calling. In addition to more third-party apps popping up to take on Skype (like the Facebook-owned WhatsApp and Messenger), phone makers themselves are taking on Wi-Fi calling with native apps, including Apple iOS' FaceTime and Google Android's Duo.

There are also phone networks that prioritize Wi-Fi calling first, like Google Fi and Republic Wireless, which offer customers cheaper alternatives to regular phone plans. To compete with this, legacy carriers that use traditional cellular technology have adopted Wi-Fi calling too. Whether it's because they want to bolster their network coverage or improve user experience, T-Mobile AT&T, Verizon and Sprint sell phones that have Wi-Fi calling features baked in.