The problem stems from how Apple activates iMessage using your telephone number –but really doesn’t offer any way to deactivate your account (while making it seem like turning off iMessage on your device is actually doing much of anything). Of course, the problem goes a little deeper… but the result is that your friends can’t always reach you by text message anymore (and they don’t even know their messages aren’t being received by way of an error).
If nothing else, this guide gives a list of the things you should do if and when you decide to move from your iPhone to an Android device.