Given the run around

I paid up my loan in November 2016 and never received a paid up letter. Every time I called I was given the run around as to how I can get the letter. Instead, I got different collection call centers calling me to collect on behalf of Getbucks and every time I call them to query this they require a paid up letter which I never received. It's been a year and this is very irritating. My credit record is in the dumps because of this.