Closed line contract billed as open line

2015/09/27 took out closed line 500mb data contract with Huawai tablet for R99 per month plus R36 for insurance. from the first month I have never been billed correctly, the deduction is supposed to be deducted on the 1st of every month, these dates have changed and never been constant. I have been billed in excess since inception, when calling to query this I have been met with unhelpful staff who are attempting to bill me *******ly out of the contract I signed with Mobile Talk and Telkom. I have been billed to date +/- R2700 in excess of what I should be billed. this needs to be rectified and refunded immediately as I am not being billed according to my contract terms. please have a manager contact me immediately.