If you've succumbed to the temptations of Android — or even Windows Phone or BlackBerry — there are some steps you need to take to make sure the iPhone friends you leave behind can still reach you. iMessage works by routing iPhone to iPhone messages through Apple's servers instead of sending them as actual text messages. Unfortunately, unless you tell it to stop doing this, iMessage has no way of knowing you've left, at least until it times out after a couple weeks, and that's beyond frustrating. Luckily, deactivating it is easy. Here's how!
Complete these steps from your iPhone before starting to use your new smartphone:
That's all there is to it. Once you've done this, iMessage should hopefully deactivate from Apple's servers and you shouldn't have any issues, nor should you have to go any further into this guide. As a side note, the sooner you do this, the better off you are. If you know you're going out to purchase an Android, BlackBerry, or Windows Phone in 3 days, do this process right now. That way you have turned off iMessage and it gives the servers a few days to actually process the request completely and boot you off.
Once you've switched to your new smartphone, verify you can send and receive text messages to someone else using an iPhone. If you aren't receiving anything back, continue on to remedy the problem...
Unfortunately, there isn't a list of steps you can perform on your own. Instead, you need to call 1-800-MY-APPLE in order to have Apple manually de-register your phone number from the Apple servers. In order to do this, make sure you have your Apple ID on hand as well as the phone number in question.
To expedite the process of your call, be sure to ask for technical support when calling in. Then just tell them you're switching to a smartphone that isn't an iPhone and you can't receive messages so you need them to manually unregister you from the iMessage servers.
If you're having issues getting iMessage to activate, get more help at the iMore forums, and for more tips, check out: