To read instructions from Apple on how to fix issues with Imessage click here https://support.apple.com/en-vn/HT204065
Using iMessage is hardly any different from using an app like WhatsApp, Imessage even shows up in separate threads.on your phone.
All Apple cares about is verifying ownership of your phone number. After that, you can turn off the cellular radio completely, remove your sim card, whatever, and Apple doesn't care. iMessages never get delivered to your wireless carrier and have nothing whatsoever to do with your sim card.
The only difference with Apple's service? Once you insert a new sim card with a new number, that new number gets activated to be used with iMessage and the older number gets deactivated. For all Apple knows, you might have canceled that number's service and will no longer ever use it.
