No paid up letter from MBD

Hi I had an account with MTN which was handed over to MBD inc when I lost my job. I paid the outstanding balance by September 2017 and requested a proof or a paid up letter in October 2017 which I never received. A few days a go I received a call from a debt collecting agency regarding the same MTN account which I had already paid up. I then explained my story and they requested proof in a form of a paid up letter from MBD. I called MBD offices on Monday I explained my story and the lady I spoke to confirmed that my account had been paid up when I requested a paid up letter she hung up. I called again the gentlemen I spoke didn't ask for any of my details but assured me that I will receive a sms from them and an email with the letter in question. Today I have been calling all morning and no answer. I am worried about my credit status at the moment if MTM continues to see me a debtor to them.