I don't think electronic interference is the problem. Cellphone towers and electronics operate at GHz and MHz, which are different frequencies than brainwaves. Interference seems to typically be labeled by a "Signal to Interference" ratio rather than signal to noise. From my understanding, noise is still inherently present even in a faraday cage, through things like thermal noise and amplifier noise, which is why the poster mentioned you still have to perform signal processing over very long intervals to remove noise.