how do i add iphone that isn't mine

JBRAVEBULL Said:

"how do I add Iphone that isn't mine"

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You don't. If it is not your iPhone, then let it be.

Is this a used device? If so, have the seller perform as instructed here, including the iCloud sign out: What to do Before you Sell, Give Away, or Trade in your iPhone, iPad, or iPod touch - Apple Support. The seller should have known better.