It would seem evident to me that sticking a piece of non-heat-conducting plastic near one's ear for a long period of time would reflect radiant heat back at the ear, so that said ear would have less cooling for that period of time, and would therefore have a slight raise in temperature in comparison to the ear without said piece of plastic.

Also, electronics give off heat. You've got batteries in a cell phone -- those are going to give off waste heat.

So you'd want to make a comparison vs. a simple block of plastic the same size as the cell phone, and also a block of material which is radiating heat to the same degree as the cell phone, but no electromagnetic radiation (other than the unavoidable black-body radiation).