Vodafone postpaid service fraud

I have a Vodafone postpaid internet plan of 450 which has a credit limit of 1400. They are sending me a bill of 4000 saying u have exceeded the credit limit. Credit limit is set for that if the consumer touches the credit limit the services will be stopped. But these people don’t know the meaning of credit limit. And didn’t even sent a single message about the exceeding limit. I even called Vodafone customer care for more that 5 times in that month and no one informed me about the exceeding limit.
Please help