Apple's iMessage is a convenient way to get text messages from users of other Apple products, without actually using text messaging. Unfortunately, iMessage associates itself with your phone number, and when your phone number hops to a device that doesn't support iMessage (for instance, your shiny new Android phone), you suddenly stop receiving your texts.
If you're lucky, your iPhone-using friends will simply get an error when they try to send you an iMessage, and can tap it to re-try sending as text. But sometimes the error never appears, and they think you got the message even though nothing came through on your phone.
If you're going to switch from iPhone to Android, you'll want to take a minute to disable iMessage before you give up that Apple device.
Disable on your iPhone
The quick way to fix this is to disable iMessage on your iPhone. Just go to Settings, then Messages, and turn iMessage off. If you have more than one iOS device, you'll have to do this on all of them. This is especially important if you're giving up your phone number—even if you reset your device, some stranger that gets your old number might start receiving messages meant for you!
Disable on the web
Don't have access to your old iPhone or iPad anymore? There's hope for you yet. Just head to supportprofile.apple.com, and log in with your Apple ID. Click the Edit Products button at the top of your My Products list, then remove the devices you're no longer going to use.
What if you have a Mac?
If you have a Mac and you still want to receive iMessages in the Messages app, you can simply remove the phone number associated with it, while keeping the email addresses you receive iMessages at. That way, if someone sends a text to your phone number, it will come through to your new Android phone, but if they want to send an iMessage, they can use your email address.
Just open Messages on your Mac, and look at Preferences. Under the Accounts tab, choose your iMessages settings. In the "You can be reached for messages at:" section, un-check your phone number. Also, make sure "Start new conversations from:" at the bottom of the window lists an email address, rather than your phone number. Now, Apple users can still send iMessages to you at your email address, and you can read them on your Mac, while texts to your phone number will reliably find their way to your Android phone.