Anonymous on "wrong postpaid reliance bill"

I m using Reliance no 9305158602 from laste10 years
And changed it to post paid number from last few months.
Even month I m getting Bill of around 1000rs. But today I received a SMS that I have to pay 5488.9 rs for this month for internet that I never used.not my out going calls also stopped.plz resolve the matter as earliest as possible.
Thanks in advance.
Best regards
A R little