Unlocking iPhone with unknown owner's iCloud password

This is what your uncle needed to do before he sent you the phone --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Until he removes the phone from his account, you cannot use it.

Congratulations, you are in possession of a phone you can NEVER use. Activation Lock can be removed ONLY by the person who locked the phone. Apple won't help you unless you have the original purchase receipt, with the phone's serial number clearly printed on it. Since you aren't the original buyer, you have a useless device. And since whoever sold you the device isn't responding to you, you've been scammed and likely own a stolen iPhone.

If you can't get your money back, which you likely can't, recycle the phone responsibly as it is unusable.

Read this --> The All Too Common SAD Reality of Buying … - Apple Community

You can't. The activation lock is an anti-theft measure.

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