contract was changed

about a month ago a guy by the name of Thomas came to my work. he said he was from cell c. showed me some specials and I said I am interested in a mobile wifi. I clearly told him I wanted a top up and no running credit he assured me its a top up for only R149 A MONTH AND NO EXTRA . yesterday I get a bill of R2034 from cell c. I phoned them and they told me your contract has a credit limit of 2000 after I asked for a top up. I then phoned thomas and he told me it was a top up but I must fone head office because he no longer works for cell c. he gave me a false number and now wont return or reply to my msgs. here is the number ***. I then phoned cell c again and said they were going to report it and gave me a ref number ***. I am still waiting for the call. I have no copy o the contract and Thomas told me I don't need it. he clearly stated to me it was a top up on the phone yesterday so who changed it. I will never recommend cell c if they treat you like this. I am trying to get my money back and just want to cancel my contract beccause cant afford that every month. I ope cell c can get back to me and resolve this once and for all.