Loosing airtime everyday without using it

Hi. I'm using Blackberry 9700 Bold and i'm on prepaid and i load BIS every month. Whenever i load airtime i loose anything from R6 to R8 a day without using my phone. i've been to Cell C shop in Richards Bay Boardwalk Zinkwazi Mall and they've checked my phone for any subscription but they didn't find any. I've contacted the Call centre on numerous occasions only to be treated by rude and uncaring operators. Please help me i'm tired of wasting my money on Cell C. This has been going on for almost a year now.