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Depends. They found one of these in New York but it’s very easy for 10s of thousands onto gather in a relatively small area. For example, New Year’s Eve, sports/concert at msg, regular foot traffic at Times Square, etc. so I think barring even antennas shenanigans, disguising it could be not impossible.

(I also understand they rarely use all active SIMs at the same time but instead rotate through in order to avoid arousing suspicion)

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> but instead rotate through in order to avoid arousing suspicion

Also to use fewer resources! Compared to the SIM cards, the radio/modems are expensive. It's cheaper to reuse one radio/modem cycling through different SIM cards for just long enough to receive pending SMSes. But not too many, or you increase latency. It's probably more suspicious to have the same modems cycling through SIM cards than to have all of them always connected.

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