If you're inside a Faraday cage, you are supposed to be electromagnetically isolated from the outside. But I was able to make a phone call and even use Wi-FI on a plane. Why doesn't this violate the laws of physics?
Simply because you are not inside a perfect Faraday cage. EM waves can propagate through the windows and various holes of the cabin.
Which frequencies propagate depends by the size of the windows. You can think to each window as a waveguide connecting to the outside. The propagation through the windows depends by the 'waveguide' cutoff frequency. Since the frequency of cell phones is of hundreds of MHz, apertures of tens of cm let the EM waves propagate through.
Estimate the exact cutoff frequency depends upon the geometry of the windows.