But for any device that producesrequires heat (toaster, portable heater, hair dryer, clothes dryer, towel warmer, dishwasher dry cycle, etc.), the efficiency of mining is infinitemitigated.
The efficiency isn't going to be perfect because the nature of the heat may not be fully fungible with the original source of the heat in such devices.
Phone absolutely not.
Thus (unless they don't really expect to scale this into mobile phones) I think it is more of a play on the fact that small changes in electricity bill are uncorrelated in the minds of the users.