Incorrect Billing and Poor Customer Care

In December 2014 I put my data sim in my cellphone because my cell data was depleted. When I checked the balance on my data sim I saw that I had current charges on it. I phoned Cell C customer service amd they told me that I send out SMS's to 1 number which belongs to my sister who shares a house with me. Why would I send her sms's when we both is still at home. I get 100 free sms's on my cell contract which I don't even use. Everybody is on Whatsapp and that is what I use to communicate with my sister. They said they will get back to me and never did. I had to phone them again and again. They told me I did send the sms's and there is nothing they can do. I don't have any record on my phone of any sms's send to my sister and she never received any sms's from me. I phoned twice in January 2015 and was told that the person that came to the conslusion that I did send the sms's will get back to me. Today 02 February 2015 the lady did not even give me a chance to tell her the whole story <br> then she put me on hold to speake to her team leader. She turns around and tell me that there might be something wrong with my phone. But this only happens when I put my data sim in my phone.