The speed at which your line synchs is dependent on many factors, all of which involve your Telkom line. Line attenuation, signal-to-noise ratio, even type of router all play a part in the maximum speed that you are able to synch at.
If your line is in Afrihost's name and you log a ticket, it is possible for them to perform a 'port reset' on your behalf, which may result in successfully synching at your desired speed initially, but if the line is in a bad condition it will probably synch at a lower speed again shortly afterwards. Unfortunatly this is mostly out of Afrihost's hands, and you will have to call Telkom (or get Afrihost to call them) to investigate and advise accordingly.
Afrihost / Telkom will bill you for whatever speed you request, but if the line can't handle your requested speed, it's up to you to either get the line sorted out, or downgrade to pay less for a slower line.