Telkom Wifi Headache

Telkom as an ISPO service provider is a headache. According to my knowledge I have unlimited (Wifi) internet usage which I pay consistently per month by debit order R485. Telkom has sent me a text message stating that I must pay R650 for over usage. What is over usage and why was I not notified of these terms prior to me taking up the contract. I am unhappy and Will Not recommend their services to anyone!!!