I'm sure you've checked and noted SA only has calls at $1 a minute and SMS at 50c.
Interestingly they used to (2015) provide free calls to SA fixed lines.
So much as it would sadden Optus, you'll need to look elsewhere for a SA option. Here's a recent article on the options.
Aldi appear to do SA and run on a quality network. Funnily enough Telstra appear to only offer 15 countries (nice if its one you want but otherwise pretty limited)