If you already have the phone you could possibly get a massive voucher from makro or game if you take out a smartphone contract though im not sure.
The cheapest call rates are prepaid imo and during peak are vodacoms allday per sec coupled with there talking points discounts. Just buy a small data bundle. If you want to tether rather get a cellc sim and a mifi router.
I ended up using the 2gig a month prepaid sim in my galaxy s and tether with that which you could also do, but you will be stuck on the allday tariff of R1.50 per min with per min billing.