Hmmmm, if I have it correctly, the long and the short of it is... Telkom's pricing model was worked out on a line rental for the voice line installed in your home before ADSL was around. This apparently covers the cost of the copper cable that enables the communication, so enter DSL, they say it is a completely different service, so it is charged separately, but because it runs on the same line, someone still has to cover the cost of the copper cable (maintenance, replacement after theft and such). So the solution is you are forced to install a voice line to cover that cost and then the ADSL gets loaded on that. (the alternative would have been to increase ADSL line rental cost with the voice portion in anyway) but I think they kept it this way to try and "add value" to the offering.
As someone mentioned earlier, the Voice and ADSL runs on the same line, but two different "frequencies" (for the lack of a better word with my limited knowledge).
So yes, what you will need is a voice line installed first, then add the ADSL capability on top of it, and then get some data to use your ADSL (capped vs. uncapped is a whole other discussion).
The Telkom simple options are pretty decent for someone just getting started. If you don't want that, when you install the voice line, take out their smallest callmore package at R15 or something a month, this waives the installation fee of the line of rougthy R700-R800. You can theoretically cancel it in the second month again and then basically pay nothing for installation. You can also take out their "contracts" where you get a router for free, then you pay it off over 24 months. It is basically nothing more than paying off the router. You can cancel it at any time, where you will then just have to pay the remainder of the router in full.
Then as soon as you have a bit of savvy with ADSL, you can start the burden of moving everything, individually or together, from ISP to ISP to ISP until you find someone that gives you decent service and deliver what they promise.
Good Luck!