Would putting it inside a microwave work? I think I recall Snowden did that but I'm not entirely sure I remember correctly.
Edit: on the other hand, I'm now considering the possibility that the phone might be recording your audio even without network access and then transmit it later when you take it outside the microwave. So you'd have to be physically far away from the microwave for the microphone not to hear you, which means the microwave and faraday cages don't add anything useful.
