I need to deregister my old number from imessages.
I need to deregister my old number but I already switched to my new number and can't receive the code
iPhone 14, iOS 16
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You can also do this online, check the steps in this article:
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Deregister iMessage online
If you don't have access to your iPhone, you can deregister iMessage online. After you complete deregistration, your phone number is removed from iMessage services. You should be able to receive text messages right away but it might take a few hours for some Apple devices to recognize that you're not using iMessage when they send you a message.
If someone sends an iMessage to your Apple ID, you'll get it on your other Apple devices. If someone sends a text to your phone number, you'll get the message as an SMS/MMS only on your phone.
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copied from: Deregister iMessage on your iPhone or online - Apple Support