We're sorry to hear of the mobile call issues you're experiencing. We appreciate the frustration this can cause and you raising it via the forums. Welcome to the community.
From what you have advised this could be down to a number of reasons, it could be the person your calling's connection, coverage in the location you are or even an intermittent issue with the handset due to age.
The first two you have no control over, the international call you made that you had problems with the second time could be down to the network in South Africa. Similarly it could be due to issues with the connection in your area, congestion, geographical layout of the area or even a fault with a local mast. These can be checked here.
If the issue is just down to the age of the handset, and as you have said it has been sent in and returned as working you could ask for a second opinion from the manufacturer. If it is is over two years old it is likely out of warranty now anyway as these tend to be 24 months or 12 months on Apple devices.