Poor service and carelessness

I communicated with MTN when I was unemployed but they still forwarded my account to debt collectors anyway.

I since communicated with Jack Phiri recently, who is a credit controller at MTN. He asked me to deposit the payment directly to MTN and ignore the debt collector. but I still get harassed by the debt collector. I have even sent an email to Jack complaining about this particular agent who keeps harassing me and promised to add additional costs to my mtn account if i do not take their calls. I still haven't received any feedback. And I also wrote to Jack saying that I give MTN authority to debit me on the outstanding balance so that we can close the account. Still no response from him/MTN but i still get harassed by the debt collector and they keep invoicing me for the call charges.