how to activate my new iPhone 7 with iOS 15

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.

deekhuntod Said:

"how can I activate my iPhone 7 with ios15"

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Troubleshooting iPhone Activation:

A. Getting Around this Error:

1. Go Here: If you can't Activate your iPhone - Apple Support
2. As it Reads: If your iPhone can't be activated or an alert says the activation server is unavailable or the SIM card is unsupported, learn what to do.

B. "No SIM":

"No SIM"*** is shown on the top-left corner of your screen. So, go here to fix that: If you See 'invalid SIM' or 'no SIM' on your iPhone or iPad - Apple Support