Someone posted here that the way around the issue was to log into every single device you have that accesses iMessages and turn off iMessaging.
However, I believe I have found an easier way to unlink a phone number from iMessage (easier than logging into each and every device you use for iMessaging).
1a. I erased the iPhone that was no longer being used (the phone number was ported to a non-Apple phone).
1b. I set it up as NEW phone, and did not sign in with any Apple ID during setup. (Skipped that step).
2. I then went to Settings, then to Messages.
3. I turned iMessage OFF. (It was on by default.)
4. I turned iMessage ON.
5. I was asked to log-in with my Apple ID, and I was shown a list of email addresses and my old number.
6. I clicked the old phone number to REMOVE THE CHECKMARK from it.
7a. I repeated the above process with two different approaches at this point:
7b. I left all the email addresses checkmarked. Testing showed that messages from other iPhones still attempted as iMessage, but then reverted to SMS after a delay. It was better than before, when the message simply never arrived.
7c. I repeated the above, and this time I clicked ALL the emails to REMOVE THE CHECKMARK from each one. I noticed that "Phone number" was blank (no number listed) and it could not be unchecked, but, hey, the number was gone. I then turned iMessage OFF on the phone.
By the time I reached THIS LAST STEP (7c) -- the overall process had worked. Text messages from other iPhones now go as SMS text messages immediately, with no delay. I'm not sure if the double erase is needed, or if the first attempt would have sufficed had I waited a little bit, or if I had unchecked all the emails during the first step.
