Debts

I have an account that I couldn't pay for 6 months was unemployed at the time and unfortunately it was handed over to MBD collectors. Made arrangements to pay the account with in a month in my new job thus far I have paid for a year. I have called in more than 3 times requesting for a settlement letter which I have never recieved. Everytime I call in the amount changes/increases by atleast 10k every 3 months. I requested for a black and white break down which I haven't recieved. This month I stopped paying, because I find pointless to pay a debt that doesn't go down. It's defeating the purpose. I would like to settle this account and move on but I don't know what should be my next move since my payments have gone down the drain.