I think you need to contact someone like Vodacom and setup the number first, I'm not sure how we here at CapeAds did it, but you have to setup the number first, you get 1 free reply when someone sends you a SMS to the premium number.
Just remember always include the "per SMS price" and that "free minutes do not apply" or you can get into HUGE kark with WASPA - actually, contact WASPA and ask them to send you the rules and regulations, so you know that already.