prepaid balance keeps dissapearing even though phone is hardly on and used

i have a prepaid vodacom sim...i keep the number loaded with airtime for emergencies Since the end of last year i been seeing the balance getting lesser and lesser even with out me making calls...I topped up again at the end of February and on checking my balance today it is nil. My phone shows not out going calls in the whole period since topping up...It is so clear to see that the service provider is taking these funds for something i am not party to...judging from the sms vodacom sends sms to subscribers inviting nthem to subscibe to a daily service of some kind.. you are invited to opt out by dailing ........if you dont opt out they automatically treat you as subscribed and debit you daily....what *******...pure *******...vodacom does not even know if the sms was received and read by the subscriber...in my case i read these sms like about once every 2 months as this is a spare phone and my friends and family know my MTN number which is the phone i use daily....mainly because i find their service far superior to vodacom...please advise why my money dissapearing