Being over charged

I have been with Mtn for over 17 years and have always had excellent service. However the last two months I have been shocked at my account being double what I normally pay. Went into Canal Walk Mtn and waited for 40 min to be told its because I am using data. Went to N1 City Mtn waited 30 min and was told its data usage. As I explained to both stores, at home I use Wifi and my data is off most of the time. Only now and then do I switch it on. I have tried Mtn via twitter no response & their customer call number and I am still waiting. Very sad and upset by poor customer service and clear explanation. As a long serving customer on business contract I am now thinking of changing networks.