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.
Hope that helps