iMessage is an instant messenger service developed by Apple. It allows users to send free text between Apple devices. But if you leave the Apple ecosystem, then you might find yourself not able to receiving text messages.

iMessage locks the phone number to the Apple ID, so when other Apple users send
text messages to your non-Apple phone, the messages are still routed to iMessage network. You end up not able to receiving text messages from your newly bought Android device.

To fix this, Apple allows you to deregister your iMessage:

• Deregister iMessage
• Deregister iMessage: Frequently Asked Questions