Milan has a OnePlus 6, but his carrier, AT&T, doesn't support voice-over Wi-Fi with it. Leo suggests a microcell, or femtocell, and tell them he's moving to another carrier if they don't give it to him. Another option is to use an app like WhatsApp or Google Voice to do it. According to the chatroom, it has to be an AT&T branded OS to support that feature.
Here's a list of the phones AT&T supports with that feature. (Thanks ScooterX)