Haw to reset a password can’t remember the phone number of the old phone
iPhone 7 how to get around the phone number reset password I don’t remember it
Earlier iPhone models
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You do not need the old phone number to reset the passcode unless Apple ID itself was a Phone Number.
iCloud uses Apple ID credentials.
If you do not remember the Apple ID Password then follow the steps given in this Apple Article -> If you forgot your Apple ID password - Apple Support
If you do not remember the Apple ID then follow the steps given in this Apple Article -> If you forgot your Apple ID - Apple Support
If you are the owner or the owner has shared all information with you then to open Activation Lock:- If it is Activation locked, read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support
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.