New tool to migrate away from iMessage

Apple had launched a new tool to make it easier for iOS users to switch to a competing smartphone platform and not lose text messages. Previously, iPhone users who switched to a rival operating system would still have their numbers locked into Apple’s iMessage servers, creating problems with text messages not showing up. Now, with the new tool, users can unregister their numbers from the iMessage system and text messages won’t be lost should you switch to a different platform.

The web-based tool allows you to enter your phone number into the system and Apple will send you a confirmation code via SMS. You’ll need to type the code into the browser and your number will be released and freed from iMessage.
Source: Apple