Billing system that does not make sense

I just got off the second call with rain regarding their billing issues. I was billed for an off peak sim which I cancelled when I upgraded to unlimited package. I am now told I am in arrears and that I need to pay for a service I did not use for over 2 months. How am I as a customer being billed for a service or sim card that was not in use for over 2 months and when I ask for a cancellation I am told that I will still be billed for a service that they have switched off. Can rain please sort out this mess because I really need to have this cancelled asap so I can get myself a new Service Provider