MTN OVER BILLING CONSUMERS

I have to say MTN is very predictable. I bought R10 airtime on the 18th at night, didn't use it al all. my phone has two sim slots so I would use my Telkom sim for data. in the morning, since I have midnight data from MTN I would change slots and use the mtn data which then stops as 6am. because I knew I had airtime i then switched my data off and started using the one from Telkom. i checked my airtime and it was still there. after 6 i went to check my airtime again and it was all gone. i am not complaining because of the R10 but I'm complaining because its an everyday issue.