All phones in South Africa are sold unlocked, and I can use my iPhone 4S with any carrier's SIM cards in South Africa. However, when I visited Australia, I found that SIM activation failed after I inserted the SIM cards for Australian networks. The person at the cellphone shop in Australia said that this was a common problem with South African phones. My wife's iPhone 5S worked fine with an Australian SIM card though.
The phone is fine. I returned to South Africa, and the local SIMs have no issues.
The South African carrier that sold me the phone has no idea how to solve the problem, as they say that the phone is unlocked. Is there a region lock that affects the iPhone 4s?