We have a contract with vodacom and have been paying +-1800 a month

We have a contract with vodacom and have been paying +-1800 a month. My wife was contacted by a vodacom agent offering her a 'migration' deal that will see the amount we pay go down.she agreed to the deal only to be supriced when the next bill came saying R2700. We emmidiately called vodacom and they promised to investigate and come back to us within 3days.. We never got the feedback and the payment date arrived and because we keep the money for paying the contract on a specific account with only R1900 in it, the debit order returned unpaid. We went into the vodashop and made the inquiry, the teller went into their system and told us the error was corrected and we paid our usual installment at the store. Now this month vodacom is claiming we owe R3500 and they are blaming us.