Billing me when my contract expired

After struggling for what seemed like forever just to port my number from my Cell C contract which expired 28/07/2015 with Autopage to prepaid I now again have an issue with Altech Autopage - luckily I am no longer a customer. I received an invoice stating that I still owe Autopgage R 216.00 for data and calls from 28/07 - 31/07. My contract expired 28/07/2015 and I was no longer able to use my phone from the morning of the 28th - how can you bill me for that? I sent several emails to enquire about this (***