Charged for data I did not use

At the beginning of June I logged a querie/complaint with Vodacom Data support that I have been charged for data that I did not use as I used Wifi during working hours and topped up with once of data bundles when I was close to my limit. The original reply was not the data has been used in May even though the data counter on my phone showed that I stayed within bundle limits. Then suddenly I jumped to \No the data in question was used in April"which is more unlikely as I have been on training during April for 2 weeks and when I was back at the office I was piled up with work and were working on the PC when at the office"