I used to have an iPhone, but I changed to an android (shock, horror!). Unfortunately Apple refuses to let me go and whenever friends message me the messages get sent as iMessages (unless they remember to specifically chnge the format). I never get these messages. They never get resent as SMSs automatically as is supposed to happen. How can I get my phone number deleted from the iMessage list. I've turned off iMessage on the old iPhone.
Disabling iMessages on an account that no longer has an iPhone can be frustrating. But there is hope. I've written a quick howto which answers this question in full. Please see it at Why are iMessages being sent to a non-iPhone device. You will find step by step instructions on how to end your troubles with iMessage.
Hope this helps!
You need to unregister your old iPhone:
Sign in to My Support Profile with your Apple ID.
Find your iPhone and click the small arrow circle on the right side.
Click the Unregister button and follow the prompts.
(Yes, these instructions accomplish the same thing as this answer. Since the answerer never wrote the instructions up as requested, I decided I should, as I feel this is an important question to have formally answered.)