in Joburg, yes.
you might not have full 3g coverage in the mountains, but it always helps just having a backup plan.
mobile data is pretty cheap i think.
Sim cards are free, just register with passport. Then buy mobile data from a supplier. You should be able to get about 3gb for around $US 50 (400ZAR)
KTM Super Moto 950, KTM Adventure 950S, Honda XR650R, Honda XR125L, Husqvarna SM610, HusaBerg FS570, Yamaha YZ250WR with rekluse