Wi-Fi calls always originate in the US, even when you're outside of the US.
Calls to US numbers are not billed (except for 411 or other premium calls).
Calls to international numbers are billed according to your international long distance plan.
Place a call to the preferred number.
Determine if the call is placed over Wi-Fi by the 'Wi-Fi' indicator (upper-left).
If you're using the iPhone X / XS / XS Max / XR, pull down from the top right of the Home screen to access the Control Center; the indicator appears in the upper-right.