phone is unable to activate

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If you are unable make a call or receive; If you can't make or receive calls on your iPhone - Apple Support

Cellular Update Failed —> If you see an Update Failed or Apple Pay Not Available alert after iOS update or restart

Settings > Mobile Data > APN [under Mobile Data APN] enter the APN pin

View and edit your APN settings

If your carrier allows it, you can view your APN settings in one of the following locations: Settings > Cellular > Cellular Data Options > Cellular Network. Settings> Mobile Data > Mobile Data Options > Mobile Data Network.

View and edit your Access Point Name (APN) on your iPhone ...

Carrier Update (Required): Released by your Cellular Service provider (Carrier Update), affects your Outgoing / Incoming calls, SMS, Data etc..

How to Update Carrier Settings

You can manually check for and install a carrier settings, update with these steps: Make sure your device is connected to a Wi-Fi or cellular network. Tap Settings > General > About. If an update is available, you'll see an option to update your carrier settings.

Update your carrier settings on your iPhone or iPad - Apple Support

If nothing helps then --> If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart

Please look at this --> If you can't activate your iPhone - Apple Support

If you are the owner or the owner has shared all information with you then, in order to open the Activation Lock:- read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support

If you have an unknown Apple ID on the App Store and Apple ID page. (See the pic below)

Learn how to remove a device from a previous owner's account.

If you are not the owner and do not have the Apple ID and Password then there is no way that you can get into this iPhone unless the original owner shares the Apple ID and password with you. If you are lucky you can get into this iPhone if they (owner) accidentally remove the device from their Apple ID.