From the CFO the listener could tell what temperature changes the phone is undergoing, but the units would not be calibrated. It you knew the temperature of the phone at a couple of different points then you could derive calibration coefficients to get an absolute temperature. If the CFO temperature dependence turned out to be repeatable between phones then knowing the model of the surveilled phone you might be able to calibrate with only one known temperature, or none if the CFO characteristics are the same for all phones (unlikely).