****** money from my contract phone

I am currently having two contracts with Vosac and I have been their clients since 2003.Recebtly I am not happy with tge services received from them. I have discovered that my contracts invoices fluctuate on a monthly basis.Wgen I followed up I was informed that the reason my invoice fluctuate is because the is a person who buy airtime advance and transfer the Money to their phone.i was even give the number that stilling my airtime. TO my surprise I am the owner of the number and never performed such services in my number .My question is why can't I be refunded my money of get proper investigation to investigate the numbers that *****ing from Me.i just can't wait for my contacts to end and leave Vodacom