Some big news for Apple this week was their unveiling of a site for removing a phone number from the iMessage system. This fixes a rare-but-serious problem in which an iPhone user can't send a text to someone because the iPhone keeps trying to send the message via iMessage instead of SMS. As a result, the text message never reaches the recipient and is lost. Usually the recipient had an iPhone in the past, but after switching away from iPhone (and keeping the same number) the number was still registered to iMessage.
So, a question you may have is "Do I need to unregister my number from iMessage?"
If you still use an iPhone, you should not unregister from iMessage. iMessage is great for sending messages between iOS devices without using your SMS allotment. If you don't want to use iMessage on your iPhone, you can turn it off in the Settings app under the section "Messages".
If you do not use an iPhone, but used to, ask a friend with an iPhone (and uses iMessages) to send a message to your phone number. If you receive the text, all is well and you don't need to do anything.
If you do not use an iPhone, and the above test failed (you didn't get the message from an iPhone user), then you need to unregister your phone number from iMessage. You probably also may have heard from your iPhone-wielding friends that they can't send you messages :)
How to unregister your phone number from iMessage
Again, you should NOT do this if you use an iPhone for this phone number.
Enter your phone number, and press
Send Code. Wait for the confirmation code to be sent to you via SMS.
Enter the confirmation code onto the page, and press
What if I can't send a text to my friend who no longer uses an iPhone?
Unfortunately (or fortunately) only the owner of the phone number can unregister the phone number from iMessage. So, we highly encourage you to let that friend know (but not via text!) that he or she needs to unregister his or her number. You can press the "Share" button directly below this paragraph to send this article via Facebook — after you press share, you can post it to your Facebook news feed (in which case you should tag the person), post it to your friend's feed, or you can send this article as a private message to your friend.