Inflated Vodacom Account

I would like to report my concerns regarding my Vodacom Account (I***/4). I have had a cellphone contract with Vodacom for over 24 months. I have noticed a substantial increase in the monthly statements over the past 6 months. I unfortunately ended up falling behind with payments from June 2015. I made the outstanding payment eventually by the end of that month. At that stage the amount was about R3600