To check the maximum speed supported by the exchange close to the area you live in kindly view the link below. You would need to enter a DSL line number to get the maximum speed supported by the exchange. The are possibilities that might make your line not to receive the highest supported speed. Mainly its caused by the distance you are from the exchange and the quality of the copper cable from your house to the exchange.
Telkom Line Speed: http://www2.telkom.co.za/checkadsl/