There are different types of debt settlement letters for the intent of eliminating a debt. A debt validation letter is not one of them. It has no function for the purpose reduction or elimination of debt.
A debt validation letter is, literally, a letter that validates the debt. It supplies information regarding the amount of the debt and where the debt originated from. By law, the collector must send this letter out within five days of the first contact. Due to the lack of enforcement of credit agencies, it's not uncommon that the letter is never sent.
You should ask for a letter upon the first phone contact from a collector. The idea is to verify that you do indeed owe the debt. It's not impossible that the collector has the wrong person. It never hurts to make sure about the validity of a debt. If you don't owe the money, you don't owe it. And if you do owe it, you'll at least have the contact information of the collector, the amount that is owed, and where it came from. The information can come in handy later in case you want to take some form of action.
A debtor that is in arrears with their creditors have options. It's best to have a consultation with an attorney in order to understand them. It could be negotiating the debt down for one lump sum or creating more favorable terms. A lawyer will have more success than someone trying to do this on their own.