Being billed for a cellphone contract not in use

I was enquiring about renewing my phone contract and I was considering to renew with Nashua Mobile(They have since been taken over by MTN) but I was offered a better deal. I then called the agent back to cancel my request to renew my contract as I was going with the other deal. The agent never called me back despite me leaving several messages for him. I received a call from Selwyn from Nashua Canal Walk to advise me that my device was ready for collection. I then informed him that I could not get hold of anyone to let them know I have decided to go with the better deal instead and he told me not to worry he will sort it out for me. That was the last time I heard from them until I received an invoice on the 16/12/14 for an amount of R173.49. I sent an email to Customer Services on the 17/12/14 ref *** notifying them of the error as I did not take delivery of the device, I haven't signed for it and that the number was not in use. I have since been liaising with Shaun Lutchman but to no avail.I provided him with an affidavit explaining the situation and sent an email on the 14/1/15. He provided me with ref ***. We in March an MTN is still debiting my bank account.