I'm no expert in the field. But from my understanding it's next to impossible with limited ressources to copy a sim card... or else we would still here people stealing cellphone numbers to do long distance (remember... it used to be a huge issue like 10 years ago with the old 1G cellphones).
So an employee would have to be good enough to reprogram AT&T's system to allow two different SIM cards on the same number, without anyone noticing, risk going for prison for spying... in order to listen to your conversations? let me know if it makes sense
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