Easy enough to do if you control the phone network, but still able to be done even if you don't control the network because the mobile/cellular phone is just a different sort of radio transceiver.
At their most basic level of operation, phones ping the surrounding area to locate phone towers (to ensure they can get a signal) and they generally like to have access to two or more towers. So your phone is essentially a small radio device that is constantly sending a signal to check it has comms, no radio silence here!
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