iCloud activation lock

See the section - Check for Activation Lock before you buy a device from someone else

in the article Activation Lock for iPhone, iPad, and iPod touch - Apple Support

If they forgot to sign out Apple ID from iPhone , so the previous owner must remove the device from their iCloud account see the article How to remove Activation Lock - Apple Support

And then follow the article What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

The previous owner must erase the device and then handover the device to you .

Now , you as the new owner must set up new iPhone with new sim see the article Set up your iPhone, iPad, or iPod touch - Apple Support

Remove or switch the SIM card in your iPhone – Apple Support

And set up icloud in iPhone Set up iCloud on all your devices - Apple Support

Make sure you are in touch with previous owner , and they must have all Apple ID account credentials , if they are having insufficient information or you have no contacts with them then device is of no use it has to be then recycled Apple Trade In - Apple

The original owner will need to unlock the device. You should return it or recycle it responsibly.

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