Bad service

Hi, this is not the first time I complain about vodacom, they are supposed to have debited their money on the 15 of rv month but they don't.. In fact they debit on the 12, and I vehemently said it to them to always debit on 15 of which is my pay day, they debit on the 12 of which by that time I'm unpaid.. This problem has been going on for quite some time now, and I'm tired because I always have to pay for interest of which it's not my fault, for how long will I be paying vodacom interest? I'm tired of paying for their mistakes ALWAYS.. PLEASE HELP