I had the exact same problem.
Make sure you unregister your phone number from any other iDevices you may have. You can do this from the Apple site or the iDevice itself.
In the iPhone message settings, have your friends turn on the toggle that allows for SMS to be sent if iMessage fails.
Start a new text message thread with your friends and have them do the same.
It's a real pain of a problem switching from iPhone to an android device, but it's so worth it. Apple just makes it really difficult to do so. I think your phone number is eventually deleted from the iMessage servers once you stop using it completely(?)