Poor service

I upgraded my contract last year February and have been paying 360 a month since then, the last three months i get a sms stating that i short paid on my account, which i paid the shortage, this month i paid exactly what i have been paying for the last year of my contract and i received another sms stating i short paid. I phoned cell c asking them why have my account increased and they put me on hold for 10 minutes so i put the phone down. They phoned back but does not talk, so i phoned again and they keep on transfering me to someone else untill the phone goes engaged. I think this is really poor service.If i have to pay more after a year i need an explanation. Please resolve this issue