MBD chase contstantly but never send settlement letters

My credit card account was settled over 3 years ago and confirmed by their staff today, yet it is still showing as outstanding on the credit bureau. I have requested a settlement letter twice before, with no response. When I requested it again today, the phone was put down. I left a call back for this company 3 weeks ago and I'm still awaiting a reply. I called again 2 weeks ago and spoke to an agent who said she'd call me back, still waiting. How do I get them to send a settlement letter so as to clear my credit rating? Please, if anyone can assist, it would be much appreciated.