South Africa in general, and major cities elsewhere in the region, has good telephone facilities. Although local calls are relatively inexpensive, long-distance calls and international calls can be pricey. Aside from public phones, there are also private phone centres where you can pay cash for your call, but at double the rate of public phones.
Local SIM cards are available across the region and can be used in unlocked Australian and European phones. Top-up vouchers are widely available.
In Southern Africa mobile phones are very popular due, in no small part, to the often dismal state of national landline service providers. Reception varies from country to country, but expect decent coverage in and surrounding most towns, but expect nothing at all out in the bush. Airports in some countries often have a counter where you can rent a mobile phone for the duration of your stay.
If you will be out in remote areas for even short periods, it can be worth renting a satellite phone for use in emergencies. 4WD rentals agencies can usually rent you a sat phone, or they'll know someone who does.