Okay from what I know that deactivated telephone line is not under your name so therefore you wouldn't technically be reactivating it, you'd be applying for a new line under your name in that house. Assuming you have never applied for a telephone or ADSL line before (in your name) then they would just come through and activate your line on the existing cables, no extra costs involved.
The only time I know that they charge for reactivation is if you didn't pay your bill on time and they suspended your line, then you have to pay something like R120.