Paid up letter.

So some time last week I went to Mr price to settle my account and I requested for a paid up letter. The response I got from them was that my account was handed over to some unknown company, when I tried to call that number it doesn't exist and they told me they will give me that letter as they do not have my details, now to my surprise that number doesn't work for 1 and 2nd when I went to pay they didn't tell me those things instead they were able to find my details and the outstanding payment. Now why is it difficult for them to give me the letter because according to my understanding they still have my details and their system has updated the payment. All I need is the paid up letter so I can clear my name from the stupid debt review