>> 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.
Not possible as in the settings you select what number (or numbers, and even an email address) to send and receive from for iMessage. If the SIM isn't in the phone then it won't be in the settings list anymore, and if another SIM is inserted you have to select that number in the settings.
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