Bad service and highly disappointed

I lost my phone last year December on the 7th December then I consulted for a new one which I got on the 15th as a migration. I was then told that I'll be paying R748 every month but to my surprise they debited my account with the amount of R2846.87. They then charged R517 on the 17th of which I wasn't using the simcard by then but had the phone with a different simcard. So I laid a complain but they haven't resolved the matter nor called to let me know how far they are.