Breach of contract

I entered into contract with Cell C early December on one of the promotional contracts. Basically this contract includes a router and I will receive 100 gigabytes per month for the price of R559. 00 per month. However, when checking my Cell C application it stipultes that I owe them over a R1000.00 for the month. When I called their helpline, their computor system indicated that I owe them just over 900.00 Rand. I held on for around 8minutes for a agent to no avail. Their services are pathetic and I want clarity on this. They are in breach of contract and this agreement will then have to be terminated.