Varnishing Airtime

Since the begining September my airtime disappears literally everytime i buy airtime once i make a single call of less than 3 minutes my airtime varnishes whenever i call mtn they say that i have subscriptions i must dial *141*5# to cancel them but when i dial that code it says i do not have any subscriptions and when i call mtn again they telling me there is nothing they can do...someone please assist as i can not keep on losing airtime like this by far i have lost may R700 worth of airtime if this can not be fixed im changing service providers