If you switched from the iPhone to an Android phone, you may have noticed a bug where people who previously were sending you iMessages can't anymore. There's a quick solution to this bug, however, so you can un-trap your messages. What’s going on is that when a user who has been using iMessage, which was released in iOS 5, switches to an Android phone and takes their number with them, their number is still registered with Apple as an iPhone. It’s a bug with iMessage that doesn’t release the number from the Apple system, making iMessage think that the user is still using an iPhone. You can fix this problem by following procedure.

Go to: https://supportprofile.apple.com/MySupportProfile.do
Log in [with your Apple ID] if not already logged in.
Click on "edit products"
Click on the "x" to the right of the product.
Click "unregister"