getting invoiced for closed accounts

Good Day

I had two contracts taken out in 2016 at the same time at the same vodacom shop,this year March(2017) I did the pre mature cancellation and paid it up and converted both contracts to pre paid now in SEPTEMBER I owe an amount on one of the numbers HOW IS THAT POSSIBLE????? i paid both same time why do I owe on one number but the other is fine? I've been on the phone for over two hours now all the contact centres are putting me through to different departments then when I eventually get to speak to someone and tell my story then its PLEASE HOLD....!!!!and the line gets dropped(WHY??) then I get to retentions and they cant open invoices and are phoning people in other departments who also cant open the invoice and I get told I'll get someone to send you the invoice MY QUERY IS NOT A STUPID INVOICE I WANT MY NAME CLEARED BECAUSE I PAID YOU GUYS HOW CAN I OWE YOU GUYS,and when I asked please can I speak to a manager they wanted to put me through to another department and I said NO i want you're manager then i get put on hold again and after a while someone else answers I was still explaining my story then GONE PHONE DROPPED AGAIN??????WHAT THE HELL IS HAPPENING THERE............??????