I missed payments on visa card, visa sent me a settlement letter, to respond within 15 days, and I did many times within 15 days but that department wanted payment in full, which I couldn't so they pretty much didn't want to talk to me about arrangements.
On the 15th day I called again and was told the account was charged off on the 28th which made 13 days... so now my account is with a collections agency.
So, I have a letter from original creditor dated example 15th giving me 15 days, then I have a letter from collections agency dated 30th, I'm thinking I have a strong case, I just don't know how to go about it.
The agency has gone against everything I read up in legislature, such as
Speaking to everyone in my house trying to dig, harassing calls up to 9 times a day
Screaming at me etc , the list goes on...

File a written compliant with the FTC, of what you have stated here.