If you place your phone in a shielded Faraday bag, no radio frequency (GSM, Wi-Fi, Bluetooth) can enter or exit. This physically blocks any secret firmware from receiving commands. When you remove the phone, assume it may have been listening internally, but cannot transmit.
For further reading, explore the open-source project (which allows you to analyze your own baseband firmware) and the research papers from the RISCURE firm on baseband exploitation. gsm+secret+firmware
This is the stuff of StingRay (IMSI catchers) and lawful interception. If you place your phone in a shielded