Mr Price Complaint

Hi I've had a Mr Price account since 2017 and I've always made payments in store and on time. In September I received a text message stating due to my recent purchases at mr price my installment was much more , on that same day I went into mr price and they advised me that I have been making purchases for R100 airtime vouchers which I have no idea of. Tried calling customer service and they kept on passing me through to different departments few days later I realized that I lost my card called up and had it blocked and was still waiting for them to get back to me with regards to the R100 vouchers. Yesterday while at work I recieved a call from a lady in collections insisting for payment and all rude over the phone , I dropped the line because I'm not allowed to have my phone with me and then I heard my manager calling me that there's someone wanting to speak to me & then it was mr price collections again , demanding payment from them after my issue has still not been resolved and no one getting back to me , just to get off from the call I told the lady that I'll make payment and she wanted to know when, how and what time. Later on that day I called mr price again and they passed me through to all different departments and I eventually spoke to the mobile department and explained to them that I've been trying to get this sorted for 2 months but no one was able to help me and why should you guys insist payment from me but they cant sort my issue. They called me back today (06/11/2019) saying that I have taken out a sim card which I dont know about , I asked them if they are able to see if I used this sim card and they advised me that the system wont allow them to. I am so disappointed in mr price how can they not check these things.