post paid bill

Hello sir I am from Nashik my complaint is about post paid billing

My idea numbers are 7722033440 & 7722009660 both of them are corporate sim with plan of Rs.125 & 2gb Internet data for 98 rupees for one month.

Last month(Aug 2014) for number 7722033440 I used all 2GB data plan in 15 days so to continue Internet use I asked customer care centre to activet new plan for me so they did it and even give me confirmation mail and message for new activated plan after this I called them twice for confirmation.

So as I was able to see that new data plan has been activated I started to use Internet again until next day suddenly my outgoing calls and data service was stopped. As I called idea customer care
They said I have used Internet without any data plan and have been charged as per their prodata basis rupees 4333
They asked me to wait until next billing date that is 6th of month
Now I have received bill of rupees 5879
And customer care is denying to make any changes to bill
Please help and guide me to solve this issue