Apple has fixed an iMessage bug that caused message delivery to fail for users who switched to a non-Apple phone. Messages from iPhone users to the person who switched to another device would continue to be sent as iMessages, instead of standard text messages. Since iMessage works only on iPhones, the person who switched to another device would not get any of the messages sent by his or her iPhone using contacts, unless they manually changed each message to send as a text message.
Apple’s fix for the problem is simple, and works even if users don’t have their old iPhone with them. Those who still have their old iPhone just need to put their SIM card in the device and turn off iMessage (Settings > Messages > iMessage > Off).
Users who don’t have their iPhone can now de-register their number from iMessage from this website. Users can key in their phone number and receive a confirmation code via text message from Apple. They can then enter the code on the website to de-link that number from iMessage.