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

Making a Wi-Fi call on a smartphone is nothing new. The ability to patch a call using a Wi-Fi connection instead of a cellular one has been around for years, with Skype being one of oldest and most popular apps to do so. 

But 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.