Hi, if they have opened a not as described case, then you have 2 options, and refusing a refund isn't one of them.
Option 1, you accept the return and send the buyer a prepaid label to return the phone, then once you receive it back you have 48 hours to refund in full the entire original payment of item cost and postage.
Option 2, you refuse to accept and don't send a label, and after 8 days ebay step in, refund the buyer from your funds, and tell them to keep the item, losing you the item and the money.
The cases are automated and not run by actual human beings, basically you have to refund, all you can control is if you get the phone back or not.
IF you receive an item that is not the phone you sent, either a different handset or no phone at all, then you can appeal the case decision, at which point a real human will look at any evidence you provide, and either they will close the case in buyers favour, or, they will courtesy refund you from their own pocket. It helps if you get an action fraud crime number before opening the appeal in supporting your claim.
It may simply be buyer dislikes the phone or regrets the purchase, and you receive the phone back in the working order you sent it in, in which case you can report the buyer for misusing the money back guarantee, and if they are a repeat offender a mark may be put on their account, restricting them from opening further return cases.
Sadly phones are one of the big scam targets on ebay, new moreso than used.