Quick fix: https://selfsolve.apple.com/deregister-imessage/
I had this problem before and now many people I know have the same problem. I have AT&T on android then switch to iphone then switch to android, switching from AT&T to T-Mobile then to MetroPCS. I noticed when I sent messages to some people they never receive it and sometime I don’t receive message either, no error message whatsoever. Well it turns out to be iMessage problem when switching from iphone to android to iphone to android … bottom line it have to do with your iMessage settings.
This link https://selfsolve.apple.com/deregister-imessage/ is apple website (apple.com) double click then right click select go to the site and use that service for free. This is a well known issue, so apple made that website to resolve your problem. However now a day it’s better to use messaging service like facebook messaging so that you can send receive message even video camera phone chat calling anywhere that have internet. Make sure you use public wifi safely, it’s ok to make call text camera chat video calling whatever on public wifi, but be careful if you’re logging into your bank account your password could leak to hackers hacking or tapping that wifi router connection, use VPN if you’re on public wifi and needed to connect to bank or other accounts that required you to type in, because unsecure connection could leak your account information including passwords to hackers, but it’s ok for facebook messaging calling video chat and so on.