I have received a letter from the law offices of Quall & Cardot, who are representing Cavalry SPV, LLC. Cavalry purchase my debt from Citibank, demanding payment in the amount $2,198.31. They are telling that unless I dispute the validity of the debt, or any potion thereof, within 30 days, which started April 12, 2017 the debt will be assumed valid by this office. Any thoughts on how to respond to this?