Vodacom takes my data

I just want to complain. Vodacom is so busy with trying to be the biggest provider in Africa but forget the basics. My phone a Huawei P9lite takes two sim cards. I use wifi at home and my data is turned off. Then this time of the month comes and I wake up getting a message bon my phone that my sim number one's data is depleted. First of all my phone is switched to use sim number two' data. So how is this possible. My data is turned off and I am using wifi. Please do not contact me or apologize. I will tell every one I know and meet about this and try and convince them to switch providers.

Regards, Corne Otto