Huh.

I seemed to recall that Snowden made guests put their phones in the refrigerator, which is the same principle. So I went and tested my phone in on the work fridge, and you could still make calls to it.

Filing cabinet, ditto. Mylar antistatic bag, (which has a conductive layer, though not very conductive) ditto.

Tried some conductive fabric, which we sell for e-textiles stuff, and which the manufacturer's datasheet advertises as providing some 85dB of RF attenuation, and sure enough I couldn't call my phone anymore.