A law firm called me for a credit card debt 5k. I told them that I would send them validation letter to see if the amount they mentioned is correct. But the rep insisted on making a payment plan on the phone. He said that they are law firm and not debt collection agency, who are supposed to reply to validation letters. He also continued that I would have to inform him within 2 business days if I agree to the settlement or they would proceed with the lawsuit. They are trying to settle for 3.5k. Shall I agree to it without validating the debt?