I'm presuming this is a phone they purchased from you?
They need to open a case for item not as described, if the phone was bought within 30 days of its last estimated date for delivery.
On receipt of your item, issue return tracked label, within 3 days. Only on receipt of your item, do you fully refund, through those case details, and do so within 48 hours.
Stick within those timelines, and your seller performance will not be affected.
@jacquil63