Deregister an old phone number from IMessage

Hello Rawbthegawd,

Thanks for posting in the Apple Support Communities.

If you no longer have access to the phone number, you may not be able to perform the steps to deregister that phone number. Check out the following information for assistance if you're in contact with the person that is using the phone number: Deregister iMessage on your iPhone or online - Apple Support

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.

Take care!

If you no longer have the old phone number then there is no reason to deregister it. Any texts sent to it will fail to send.