Is Telkom really SA's leading network?

Telkom has the best deals hands down! No other network can compare but why is there no signal in 90%of JHB, I work in the city central, you would think this would be the epicentre of amazing signal but it is so ****, I cant even make a clear phone call from my office, my question is, how is this possible that the biggest network in the country has such bad coverage. Please fix it!