there is NO power switch in phones. My first GSM phone was something like Nokia 2110, and even that had no power switch.
Cellphones have a SLEEP mode, they NEVER turn off. Just like laptops (there is always at least one processor running, EC) and PCs (since ATX).
You can compare it to a turned off PC. In principle its off, but network card is still linked with ethernet switch and listening for WoL packets.
Some of the confusion probably arises from the distinction between power-down and standby modes. If anything but the supervisory processor that monitors the power button were active with the phone turned all the way off, it would be noticed, and give rise to a substantial (and well justified) outcry. Under normal conditions, that can happen only if the phone has been hacked.
However, the idea that an unmodified or unhacked phone can be made to behave this way is sheer tinfoil hattery. You're not allowed to bring anything even vaguely electronic into a SCIF, but that's because the whole idea behind a SCIF is to have a room in an otherwise-unsecure facility where even the vaguest potential security risks are aggressively countered.