Hello livinginsd,
Thank you for using Apple Support Communities.
We understand that you are wanting to know about making Wi-Fi calls from South Africa to the U.S. You can take a look at the following support article on how to make a Wi-Fi call from your iPhone.
Make a call with Wi-Fi Calling
There should be no difference in making a Wi-Fi call from South Africa, than it would be from the U.S. as long as you have a Wi-Fi connection. You should contact your carrier to make sure that they support Wi-Fi calling. They could answer any questions about charges as well, although typically there are no charges for Wi-Fi calling while traveling abroad and calling U.S. numbers. Calls to international numbers are billed based on your wireless carrier plan.
We also have some helpful articles to share with you, that address using your applications while traveling, and avoiding any additional charges while traveling and using your device.
Choose iPhone settings for travel
View or change cellular data settings on iPhone
About mobile data roaming options for your iPhone and iPad
Best regards.