Mtn *****s airtime

Today I loaded R60 airtime and in a matter of 1 hour I had R30 left I had not used the phone at all. I made a 7min phone call and then had R10 left. This was not the first time this has happened as the same thing happened when I loaded airtime on Saturday. I phoned the help line for MTN and they checked my records. The lady told me that I had been on the Internet and used R22 and am currently on the Internet to which I told her I had not and can't as I am on the phone. She was very stroppy her name was Joyce and she told me it is not a problem with their provider and that I am using my airtime up. I demand to be reimbursed the money that was ****** from me and the problem to be fixed. Being told to switch my mobile data off is not a solution. If this is not fixed I will be moving to another service provider.