Faraday cages do not block all signals. From wikipedia:
> A common misconception is that a Faraday cage provides full blockage or attenuation, this is not true. The reception or transmission of radio waves, a form of electromagnetic radiation, to or from an antenna within a Faraday cage is heavily attenuated or blocked by the cage. However, a Faraday cage has varied attenuation depending on wave form, frequency or distance from receiver. Near field High powered frequency transmissions like HF RFID are more likely to penetrate. Solid steel cages provide better attenuation over mesh cages.
> A common misconception is that a Faraday cage provides full blockage or attenuation, this is not true. The reception or transmission of radio waves, a form of electromagnetic radiation, to or from an antenna within a Faraday cage is heavily attenuated or blocked by the cage. However, a Faraday cage has varied attenuation depending on wave form, frequency or distance from receiver. Near field High powered frequency transmissions like HF RFID are more likely to penetrate. Solid steel cages provide better attenuation over mesh cages.