Why is a debt collections law firm giving me the run around? HELP!?

I offered to settle an "old" debt, purchased by CACV of Colorado, LLC, who have a local law firm representing them now. The debt is over 6 years old, and we've been "chasing" it for the past 2 years to establish payment arrangements. Now, since last August 2006, it was with a different law firm (still CACV, though), and we establish a $100/mth payment plan, balance $1752.00. Okay, our last payment of $100 was sent back to us in August 2007, with a letter stating they (previous law firm), no longer represent us, call this number for istance. Well, call number, verify balance is $1342.00, ready to settle for $1150.00, after 3 days CACV ok'd settlemt, I just want letter b4 paying it to verify accepted. Everyday I've called to request letter faxed before paying, all say they will fax, never do. I've left messages with the lawyer, will not rtn phone calls. The letter they did fax is completely wrong, stated balance and settlement offer. I want to resolve this debt!