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It's a refurb.

You give them a used phone, they give you a used phone in return. You couldn't possibly expect a new phone.
 
When I was sent a new iPhone it was in a small black box. Don't know if that means anything or not.

All replacements at the Genius bar come in white boxes (whether new or refurb). All mailed replacements sent via AppleCare come in black boxes (whether new, repaired, or refurb).
 
You should be able to tell whether or not it is a refurb from the serial number.

Usage doesn't prove it's used, any more than seeing 20 miles on the odometer of a new car proves it is used.
 
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