http://9to5mac.com/2014/11/09/how-to-deregister-imessage-phone-number/
Apple link to deregister iMessage
If you’ve left your iPhone for another platform but have missed text messages, it’s most likely because your mobile number is still registered to the iMessage service.
If you still have your iPhone, this is probably the easiest way to turn off iMessage.
Most people unfortunately don’t realise this problem until they have already switched to a new device. That’s why this option will likely be more popular from this website.
From personal experience when I moved from away from iPhone a few months ago, I contacted Apple Support via online chat and had to request my number be removed from the iMessage servers. I don’t think many people were aware of this extra step needed so this online tool is a good step for Apple, even though it is for people who have left their Apple device.
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