I recently transitioned from an iPhone to a Nokia flip phone. While I can make and receive calls and send texts from the flip phone, I'm unable to receive messages that others send me. The strange part is that I still get those messages on my MacBook! My old iPhone is completely out of commission, so I can't use it for any troubleshooting.
I've also tried the deregistration tool on Apple's website, but it doesn't work because I can't receive the verification code they send. And when I click on "didn't receive code," it just leads me to a page full of unhelpful information that doesn't solve my problem.
3 Answers
If you still have access to your Apple ID settings, you could try unchecking your phone number in the Messages settings on your MacBook to see if that works! But it might not be possible without an iPhone to access those settings directly.
If you're having trouble online, you might want to head to an Apple Store. They could help you with deregistration in person.
You should try going to this link for deregistering iMessage: https://selfsolve.apple.com/deregister-imessage/. Just follow the instructions there, and it should help you out!
I tried that, but the verification codes wouldn't come through. I refreshed a bunch of times but to no avail.

I only have my MacBook now, so I'm not sure if I can make those changes from there.