If the trend is more towards ADSL lines it is probably because data is becoming more important than voice.
However, the growth in ADSL subscriptions could actually be under threat as we speak.
As a person who has never had an ADSL line, I find no reason why I should get it when there are other options through which I can get broadband. The increase in ADSL lines was probably because ADSL was considered as the fastest and most steady type of Internet connection a few months ago. However, a lot has changed since a few months ago. There are other ways of connecting securely with high speeds.
If Telkom does not bring its ADSL line subscriptions down soon, they will not be getting new customers. Right now they are relying on companies like Mweb and axxcess to bring in more subscriptions for them with their lower per gig offerings. It is not Telkom's offerings that are bringing in more customers.
People are basically forced to pay R130/mth + to Telkom so that they can get a service from other companies.