First National Collection Bureau offered a settlement then did not follow through on a written aggreement of said offer Sparks Nevada

I received a "formal" looking letter from said agency to offer a settlement on a bad debt. The offer was one I could afford and I was anxious to settle the debt and help repair my credit.  After I called and spoke with some one called Marina Navarro who claimed to be an "Account Executive", I agreed to the terms in the letter which offered a 50% of original debt settlement offer and sent the 1st of 6 monthly payments. The payment cleared and I have proof.    

About a week later, I recieved a confirmation of payment letter from this "Account Excecutive" but the letter did not reflect the 50% settlement, as agreed, but rather it showed the original outstanding debt.  I called and spoke to someone named "Adam Green" who claimed to not know who Marina Navarro was but said he worked for the company.  I expressed to him that the confirmation letter did NOT state the correct settlement amount. He stated the letters are computer generated and that the computer does not "know" about the 50% settlement and therefore would just reflect the original debt.  I asked why.  He stated when I was done paying the 6 monthly settlement aggrement, then a letter would be sent to the credit bureaus to show the debt was satisfied. This seemed "fishy". I became concerned that this "agency" was bogus. 

A few days later, I called the agency again, trying to talk to Marina Navarro.  The call was made during the stated business hours.  I tired to get through more than once and each time the call was dropped with a recording saying, "We can not take your call at this time.  Call back later."