iOS: There are always a few hiccups when you jump ship from one platform to another. If you're switching to Android from iPhone, for example, don't forget to disable iMessage.
Some phones will prompt you to do this when you switch over, but if not, you might notice you're not getting messages from iPhone users on your new Android (or other non-Apple) device. Chances are, you're still logged in somewhere: Your Macbook or iPad, maybe. To fix this problem, simply unlink your phone from within the iMessage app, then disable it. Or check out the link below, which helps you deregister your number with iMessage altogether.
Deregister iMessage [Apple]