An electromagnetic wave is made up of alternating electric fields, a charge exposed to it will experience forces regularly changing in direction. For water molecules, which are dipoles, the net effect would force the molecules into rotation.. These agitated water molecules then posess heat energy to transfer into the food, thus cooking it. The key note here is that the microwave acts as a protective box, directing the waves directly at your food. Microwaves operate at a frequency of up to 300 GHz.
Cell phones cannot direct all that energy straight into a tiny popcorn kernel since it is not in a closed box like a microwave. Additionally, they do not emit enough energy (they operate at a frequency of roughly 2 gHZ) to heat up the water in the kernels to make them pop, and if they did, they would probably boil the water in your hand while you were on the phone and eventually make your hand explode!