MTN does not need a license to operate on the WiFi frequency. Its the unlicensed band at 2.4GHz.
The thing about MTN and Vodacom is that they are both digital. They dont actually send their calls over the internet (a publicly routed network) or use the Internet Protocol. Thus, their traffic (calls) is not classified as VoIP. They have their own private licensed-frequency networks.
MTN seems to have a newer technology threshold - they seem to bring out the newest stuff first and seem to have the latest technology. However, Vodacom concentrates on new products within the scope of their current offering. They also have more coverage and the most subscribers by far (due to them being first and a powerful initial advertising campaign -- yebo gogo)
In addition, VoIP (excuse the high level stuff) uses packet routing/switching while cellphones/telkom use circuit switching which means a dedicated path for traffic. VoIP uses the available bandwidth more efficiently. Due to the nature of the internet, it is possible to deploy VoIP at a low cost or at no cost. This, in essence, is the distinction between normal voice and VoIP.
Due to licensing restrictions, only Sentech can provide wireless internet services in South Africa - no matter what anyone says. All the other companies are jumping on the Sentech bandwagon (the wireless craze) in order to market their hotspot offering(s). MTN is just using this hotspot wireless thing for publicity. I doubt they will make a lot of money out of it.
Lastly, the reason Sentech got the 3G license is that they are using a "private" 2.5GHz frequency band which means its a lot easier to regulate, manage and most importantly, control. WiFi and 802.11 use a free-to-use band and as such, anybody could cause interference and also provide similar services. So you can understand why we have the system we do. Perhaps things will change with 802.16 - only time will tell.