MTN - Threats

I am utterly dissapointed in the level of service I am receiving from MTN. I took a contract out with MTN about a year ago and 3-4 months into the contract I changed banking details. I was under the impression that Captiec (my new bank) has taken over all debit orders from my previous bank FNB (which they had done with all other debits). I do not check my banking and was under the impression that they had deducted the monthly payments for the contract as per norm. A year later MTN came back threatning that I owe them over R12 000 and also threatning to black list me. They have all of my personal details and not once did they ever contact me to inform me I have skipped a payment or failed to make payment. Why do they come back a year later and suddenly have my details after it being on R12000 to contact me for payment arrangements. They failed to SMS me, call me, email me or even send a post to my home regarding this matter. I will not and cannot afford to pay this amount and I also cannot afford to be black listed.