WhatsAp question

If I’m in Europe, and have a WiFi connection, I know I can call the United States for free….does the person on the other end also need to be on WhatsAp.

Yes both ends need the app, that’s the way I have used it USA to Kuwait and Canada. To reach regular phones, I use the KeepCalling.com app which is a paid service but very, very inexpensive. Literally a few pennies per minute.