Data goes missing on Netman contract packages

I have subscribed to the 2 year netman package (WI FI). You get 20 GB free per month . On the 28th of every month it goes onto the nest month with your 20GB. Some times the data just disappear. This time I was not at home (living on the farm) no one around and the whole 20GB according to MTC was used on downloading movies or something similar but no one was at home. MTC is corrupt randomly stealing data from customers on contract. All my friends with the same package have similar experiences.