Data sin for R199

I took out a data sim 20gig day and 20 gig night for R199 in september. Every mothh they bill me more than R300. I just recieved the sim not a modem if i go in to the branc thet say i must phone call center because i took the contract over the phone and when i phone call center to ask why i didnt get my contract yet and why is my accout R300 not R199. They say it will be sorted out and someone will phone me back up to now its not sorted out and are still waiting for that call back. When i only pay R199 that i must pay they put the sim off so they force me to pay over R300 i feel that is *****. I wanted to take out a cellphone contract from them but now i feel i will never do business with them again.