With regard to the above i hope this will be of assistance to you;
It is correct that you have to have the Telkom line to faciltate the DSL service, we have long waited for the implementation of LLU (local loop unbundling) and maybe the fact that this may lead to a 'naked' DSL service ie, DSL without the analouge line rental. Telkom have been dodging this bullet for a long time. Last week at the MyBB conference I chucked the question at Steve Lewis ( Telkom ME) while he was doing a panel discussion, his answer was as expected that the analouge line is the delivery medium for DSL and there are no plans at this moment in time to offer naked DSL !
There is however cost savings to be made by retaining your DSL and using a VoIP service, Rentals are typically around the R50 per/month for the service and savings are made on both calls and the implementation of per/second billing, your once off investment would be the price of an IP phone. You may port your existing Telkom number to a provider for a once off charge as well. You savings will really depend on your call volumes that you have.
The last solution is to use wireless 3g and dump the Telkom one time, wireless speeds are increasing, bandwidth is coming down in price as well, so this can be an option. Be awear that 3g will allways have a higher latency than DSL and VoIP does not like high latency. More importently you need a very good connection, if your signal is not strong the VoIP will give you issues. Even in areas that have very good 3g you have some bad days ! 3g is not stable all the time, on these bad days you can get issues. So you must investigate and test in your area before committing to a 3g option.
Paul