Something must be in the water, because I just got ANOTHER flare up of people asking me about Faraday bags to prevent phone tracking.
Short answer: A Faraday bag probably doesn’t solve your problem; they require great care to use effectively. and your phone can be tracked as soon as you remove it.
But if you DO need one, you want one that actually works. Here’s a short writeup I did a while back on the theory and practice of testing them:
https://www.mattblaze.org/blog/faraday/
TL;DR: Science is fun.