Charged for BIS by mistake - Now no refund by MTN

I upgraded from a Blackberry to a Nokia on 17 October '13, as per my upgrade I no longer had access to Blackberry Internet Service as I have a Nokia. MTN however continued to deduct the monthly fee from my account for BIS. I pay my phone via monthly debit order and do not scrutinise my statement as I assume that everything is set up correctly. When I eventually noticed that MTN had been charging me for BIS and when to sort it out, I was informed that I reported it too late and that I would only be refunded for two months instead of all BIS charges to date. So technically I now have to pay for their billing mistake? If that be the case, I will cancel all contracts with MTN and rather take my business elsewhere as this type of customer service (or rather lack there of) is unacceptable and I will not tolerate it.