When is it okay to not pay a debt?

I have been trying to clean up my credit record by paying off debts that had accrued over the years.I have tried on several attempts to get this particular bill, made in 2003, taken of, but I always received the runaround. I went into the bank and everyone blew me off because the debt had been turned over to a collection agency. About 3 years went by. I tried to contact them again to see about getting this debt paid. I spoke with someone by phone, but again I was blown off because the debt had been turned over to a collection agency. I contacted this particular creditor about a year and a half ago and inquired about how to get that debt paid. This time no one ever returned my calls or emails. October 2010 I contacted the bank. I spoke with someone who said they would contact the collection agency to let them know that I am interested in paying off the debt. The collection agency should then call me to set up a payment arrangement. I never heard from the collection agency. The only collection agency that I dealt with said they no longer have the loan. The original creditors turned the loan over to another agency. In April 2011 I contacted the bank again to attempt to get this debt taken care of. I spoke to the manager. I was told that a voucher would be sent out in order for me to make a payment. I have not heard anything from anyone. It's crazy to have to run people down to give them MY money. What are my legal rights here?