"Do you have a cellphone?" (formal) = "Tiene usted un celular".
Yes yes yes, I know it's more common to have "Usted tiene" but that is also more ambiguous of a statement vs a question. "Tiene usted" leaves no question of it AND it's presented in the lesson as possible and I was still marked wrong saying I should have just used "Tiene". That is a real basic mistake for a website that I trust to teach me more Spanish than I have learned on Duolingo, especially when Inma already stated below apparently you added it to correct answers a year ago.