Telkom upgrade

i have upgraded my contract last month and paid R 325 as per new deal now i received an invoice for R 350 for this month . when i check it it says i used R 25 more on calls.i have trying to call Telkom without success , please set my spend limit to be at R 0.00 i dont need additional costs to this contract pleaseeeeee. R 325 as the agreed contract is all i need.