Most android phones are.
Although there are many apps which can make calls on wifi (example: Skype, Facetime, etc), the other person has to be using the same app.
Wifi calling, per se, is a native feature of the phone. Both Apple and Android phones support this feature. Both phones do not have to be using wifi calling to connect and have a conversation. It's just a feature that you can use on your phone when wifi is available and your cellular network is not. It connects your phone to your carrier through the internet.
Note: All phones wifi feature is not supported on all carriers. Specifically, some MVNO's do not support native wifi calling, regardless of what phone you're using. That's one of the reasons that an MVNO is less expensive. The major carriers generally support wifi calling on most phones which offer it.
https://www.howtogeek.com/234608/how-to-enable-wi-fi-calling-on-an-android-phone/