All buyers have a 100% feedback rating.
You only have 45 days to get your problem resolved. Your best option is to ask the seller for a complete refund. Actually, ask that he also refund the cost of returning the phone, but that's just a bargaining chip.
You will have to return the phone using CONFIRMATION OF DELIVERY. This is Regular Parcel (domestic) , Expedited, ExpressPost or Tracked Package.
There will be a bar code on the shipping label.
If the original payment was over $250 you must also purchase Signature Confirmation.
The seller should refund in full (price +shipping) on receipt. If he doesn't, open a Paypal dispute* and give the DC number in the dispute. PP will refund you and go after the seller.
*If over 30 days have passed since the end of the auction, open the Dispute immediately. A polite note to the seller can assure him that you only did this for your Buyer Protection. If he refunds promptly, it will not impact on his seller account.