Data being used even when wifi is on

[AC Question]

I am using Redmi 2 android mobile with mobile connection of BSNL in India. I was using BSNL data plan when on tour and after returning to my home, switched to BSNL wifi by activating it and switching off data use in settings on or about 07-012-2015. However my mobile continues to charge 3 paise/10KB as data usage since 07-12-2015. My repeated complaints to BSNL could not stop data charging and as per its representative systems at BSNL are working properly and there could be problem with my mobile. I rebooted mobile, closed it fully and reopened many times but problem continues. Can someone guide me to stop charging of data use by BSNL, while I am fully using wifi only. Previously I did not face such a problem on this mobile nor my other [non-Redmi] mobiles/tablet with BSNL connection are having this issue.