There are a number of ways you can use the Internet (to which you may just happen to be connected to via wi-fi) to make audio or video calls but they are not “Wi-Fi Calling” as referred to by Seve and myself.

The term is confusing but it refers to specific technologies (explained here on Wikipedia under General Access Network).

Essentially, GAN allows cell phone packets to be forwarded to a network access point over the internet, rather than over-the-air using GSM/GPRS, UMTS or similar. A separate device known as a "GAN Controller" (GANC) receives this data from the Internet and feeds it into the phone network as if it were coming from an antenna on a tower. Calls can be placed from or received to the handset as if it were connected over-the-air directly to the GANC's point of presence. The system is essentially invisible to the network as a whole, GAN is used to allow UMA-compatible mobile phones to use Wi-Fi networks to connect calls, in place of conventional cell towers. This can be useful in locations with poor cell coverage where some other form of internet access is available, especially at the home or office. The system offers seamless handoff, so the user can move from cell to WiFi and back again with the same invisibility that the cell network offers when moving from tower to tower.