Airtel
Wrong billing

I have taken mobile connection from airtel with I-PHONE.

they had offer of free 599mb usage of GPRS free with this connectin.

while doing initial provision, one of their employee made a mistake.and i m being charged every month

for Rs 600 for the same, which is free.

several times i took yhis issue to Airtel Retail Outlet in santacruz from where i have taken this connection

but nobody is taking any action.

And every month i m being harassed by them for making this extra payment.

plese help me out with these.