It is extremely important that BEFORE you switch from iOS to Android that you turn iMessage off (sign out) on *all* of your devices associated with your Apple ID - iPads, iPhones, iPod touches and Macs.
This needs to be done before you pull your SIM card out of your phone. If you miss a device and don't fully sign out, you'll continue to have this problem.
This cannot be stressed enough. Sign out of iMessage on all your devices. You should have your SIM card in your phone when you sign out on your iPhone.
See, Apple's database links your mobile number to your Apple ID when you sign in to iMessage the first time from your phone. This is how you're able to receive iMessages on your iPad from other Apple users who sent the iMessage to your mobile number (the iPhone looks up your number, sees it is associated with an Apple ID, then sends the message to Apple's servers instead of via SMS). The only reliable way of clearing this is to sign out of iMessage on your phone and sign out on every other iOS/MacOS device.
Now, there are other causes, too. Your friends may be sending messages to your *Apple ID* (like an old Blackberry PIN) and not actually your mobile phone #. Those will continue to fail unless they're using your phone #. And if they've turned off SMS on their devices, like stated above, there's nothing you can do but encourage them to turn it back on.
Also, if they're sending you a message, they can force it to go to SMS, which may speed up the process of their devices realizing your phone number is no longer associated with an iPhone.
But, and I can't stress enough, every device that uses your Apple ID needs to be signed out of iMessage first. Then, once you're receiving SMS's from iPhone users, you can turn back on iMessage on your non-iPhone iOS devices if you want (via your Apple ID, but this time there will be no phone # linked to it).
Posted via Android Central App