I downloading the yellow trader and perhaps that will help. I could not find the place where you can see what prepaid, topup or even contract data bundles they have. Perhaps this is all hush hush and you need to log in first...
I am not sure where you came up with this, but prepaid has always had OOB, the only difference is you could control it by not loading massive amounts of airtime for voice calls at a time. On a contract you cannot control it, since if you go over your limit it just accumulates the charges and "bills" you at the end of the month.
If this is something new, that would be great.
Example : You have R50 airtime and a 1GB data bundle, you then deplete your data bundle and continue browsing the web. Once you use 25mb R2x25 your airtime is also depleted and you can no long make calls or browse the web, you then buy another 1GB data bundle and R50 airtime.
Another scenario is once your 1GB runs out, you notice and buy another bundle and still have the R50 for voice calls or you have the sim in your Ipad and it only has data bundles, no airtime in which case once your data bundle reaches zero you need to recharge.