BSNL — bill for broadband extra usage which i did not use.

Address:

Sangrur, Punjab

my landline number is[protected] i was using bsnl broadband 1gb plan for few months i used stay away from home and used broadband very less but i was always getting extra bill of around rs1000 for few months but i did not had the time to investigate and i thought that i might have used broadband or some background service might have downloaded. but this week i took some time to investigate and i registered on selfcare.ndc.bsnl.co.in and viewed my usage stats and found that within first 15 days of feb 2014 i have used more than 3gb data but i have not downloaded a single item not even a 2 mb song from my home connection i got suspicious and checked dates and found that on 1feb my broadband was used around 2gb and point is on 1feb i was not at home my laptop was with me and modem was not connected at all. modem was packed and placed in almira. then how could i use interenet. but i am getting bill for that i got few previous dates on which i was not at home and it is showing broadband usage on those dates such as in january. now please suggest me what can i do.