You are correct. SAIX and IS are two of the main backbone providers in SA. IS has it's own international connectivity via satellite and SAT3 which makes it a backbone provider. Verizon/MTN Business also has satellite connectivity making them a backbone provider too. And now with Neotel having access to SEACOM, that makes another backbone provider!I see. Well, I always assumed that IS and SAIX are two backbone providers (in the non-physical infrastructure sense) and that other ISPs (Axxess, Nexus, WebAfrica, OpenWeb, etc) would buy their bandwidth/usage from them.
Telkom's ADSL infrastructure has nothing to do with them being a backbone provider. Backbone providers very often don't even have last mile connectivity (local loop).