3rd party collections or not ?

[BKHELP138]

I have been getting calls and letters from some companies I owe money. They were 1st from the lender themselves then a 3rd party. When I contacted the 3rd party they say they are collecting for ABC bank but on the letter they want me to pay them and not ABC bank. The reason Im asking this is I want to settle with some of my lenders. But I do not want to pay a 3rd party only them. If they have sold the debt then oh well. How is one to tell wether its sold to a 3rd party or a 3rd party collecting on the banks behalf ?