I think you will just have to wait for the buyer to post the phone back.
I really would not message them and chase them - they have a deadline from Ebay which they will have to stick to if they want a refund.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)