If you've switched from iPhone to something else, like Android, and aren't receiving text messages, it could be that your phone number is still registered with iMessage. If that's the case, Apple has an online tool you can use to deregister with iMessage and get your SMS/MMS working properly with your new phone.
How to de-activate iMessage online
- Go to http://selfsolve.apple.com/deregister-imessage from any web browser.
- Scroll down to the second section labeled No longer have your iPhone?.
- Enter the phone number you'd like to de-activate from iMessage and click Send Code.
- Check your current phone for a text message and enter the code that you're sent.
- Wait for confirmation that iMessage has been de-activated.
How to re-active iMessage when you come back to iPhone
If and when you later switch back to the iPhone, all you have to do is turn iMessage back on in Settings.