Wifi calls on supported phones, or Digits calls via wifi data, are free to US numbers.
If using regular wifi calling (on supported phones) just be careful that it IS actually in wifi calling mode, as otherwise it will get charged as a regular phone call. I've had a couple of charged calls because incoming calls came through via roaming when I thought wifi calling was on (luckily in 20c/minute countries.)
Digits will work with the phone part off and wifi on (or on a non-phone device like a tablet), and is safer for that although I've found the audio quality isn't generally as good.
The free data roaming doesn't apply to voice calls anywhere, although the flat voice roaming rate is not bad compared to older phone plans I've had.