This can happen if you used iMessage on your iPhone and then transferred your SIM card or phone number to a non-Apple phone. If you didn't turn off or deactivate iMessage before switching to a non-Apple phone, other iOS devices will still try to use iMessage to send text messages to your phone number. Follow the appropriate steps below to resolve the issue. Likewise, if you can't send someone a text who used to have an iPhone, having them follow these steps should resolve the issue.
If you still have your iPhone
- Put the SIM card you transferred from your iPhone back into your iPhone.
- Make sure that you're connected to a cellular data network (such as 3G or LTE).
- Tap Settings > Messages and turn iMessage off.
If you don't have your iPhone or if you can't turn off iMessage
These steps won't deregister your phone number with iMessage
- Logging in to My Support Profile and changing your password or removing your phone number
- Texting STOP to 48369
- Resetting your network settings
- Changing your Apple ID password