Loop Mobile
Changing of Post paid connection to Prepaid

I took a Loop mobile Sim card and Post paid connection (Yearly Rs. 500/- I am paying and bill as per using) from Loop mobile gallery, Vashi, Sector -17, Navi Mumbai. But they are cheating at the time of billing. Full month I am in offshore where I am not getting any signal and my mobile is switched off, still they are sending bill of some amount and they are not showing me bill details also. I went there four times but they are not changing my connection from postpaid to prepaid with same number.