If you switched to a non-Apple phone and aren't getting SMS/MMS messages, you might need to deregister iMessage.

When an Apple customer sends a text or picture message, Apple determines if the recipient has iMessage enabled. If so, Apple sends the message internally as an iMessage and it is never sent as a SMS/MMS message.

This can cause issues if you have switched from iPhone to Android. Because iMessage is not available on Android, you would never get an iMessage.

Apple has provided a self-help tool to deregister from iMessage for customers who have moved to an Android phone, which can be found here.

Once deregistered, you should be able to receive text messages right away but it might take a few hours for some Apple devices to recognize that you're not using iMessage when they send you a message.