Contract due to expire and Top Up increased

Hi,<br> <br> I have a contract due to expire in November 2015. The device cost is R139. I have to allocate an extra amount for calls and data. What i find strange is that the beginning of 2015 that value was R0. No calls could be made. I then increased it to R100. My monthly bill consists of R199 for 3G contract, R139 for phone and the rest is top up for the phone. Bill for past for months Oct R638, Sept R530, Aug R388, Jul R388, Jun R344. It for some reason increases on its own. As the limit is reached. 4 days ago I wanted to see what the top up value is set at. It was now set to R300. When I asked them to bring it down to R100. The system does not allow them to. <br> <br> Question does the system do this once the contract is due to expire.