If the iPhone (or whatever Apple device you're logging into Apple servers with your Apple ID) is not connected to the servers, then the messenger service knows you are offline. If you take the SIM out, the iPhone will still be able to receive iMessages sent to your user using WiFi or another data SIM.
Not sure what would happen if you swapped the SIM in an iPhone and someone tries sending an iMessage using your old phone number instead of email address.
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