Incorrect data billing

On 15th May 2015 I addressed an e-mail to MTN's Customer Care e-mail address advising that I have received my statement dated 11th May 2015 however I fail to understand the high data costs as I am connected to WIFI during weekdays from 07h00 - 15h30 while I am at work. MTN has confirmed receipt of the query and provided me with reference number [#***0]. I have since then not received any response, despite the fact that I have sent 2 further e-mails requesting feedback. Please take note that my average billing cost is normally about R850 but last month it went up to about R1 500.00 and this month to approximately R1 700.00 - all excessive data costs, which I fail to understand due to my wi-fi connectivity during office hours. .