How to Deregister my old number from my iCloud?

See this Deregister and Turn Off iMessage - Apple Support

Remove your phone number

If you don't want calls or messages for your iPhone on your iPad, iPod touch, or Mac, you can remove your phone number. There are two ways to do this:

• On your iPhone, go to Settings > Messages > Send & Receive. Sign out of your Apple ID account. Then go to Settings > FaceTime and sign out of your Apple ID account.
• If the phone number that you want to remove belongs to an iPhone that you can't access, change your Apple ID password. This removes all phone numbers from your iPad, iPod touch, and Mac. Calls or messages for the removed numbers won't appear on those devices anymore.

If you have questions, contact Apple Support.

FaceTime and iMessage - Link your phone number and Apple ID for use with