Do you have to pay for Wi-Fi calling

No, there are no additional charges for Wi-Fi Calling.

My carrier (AT&T) doesn’t bill separately for Wi-Fi calls, even when I make a Wi-Fi call when not in the US (e.g., from Paris, if I ever get there again). For domestic calls I wouldn’t know, because my plan includes unlimited calls and texts.

Most carriers bill for WiFi calls the same as regular cellular calls. They still go through your carrier's switching equipment, they just get there via the Internet as opposed to cellular.