The mention the Antenna System & Supplies Inc.’s TRJ-89 jammer, which is able to block cell service within a 5 mile radius. That should quiet things down.
There is usually a supercap or coincell battery that powers a very low power 32k clock, which basically keeps track of time while the device is “off.”
Last I checked, it was a requirement by US law that phones emergency camp on a network even without a SIM card. This allows you to call 911. So even when it says no SIM, it is looking for and camping on a network.