Anonymous on "overcharging every month"

I took a postpaid no from Vodafone. a basic plan of 199 Rs per month. The idea was to keep a spare phone in the house for incoming calls when landline is out of order .
These people charged me 2000 Rs ever month, I paid twice, assuming that there is some virus in my phone due to which internet is being used.
I took the phone to the store, had that checked. Then switched off the phone and forgot about it for 40 days and will you believe I got a bill of 2698 Rs.
And now they hound and harass me everyday for the money. I feel so helpless. what should I do?