Is power the only relevant variable? What about wavelength?
Cell phones are not the problem (well, not the microwaves at least).
You can measure the energy of one photo using this equation.
Microwaves at 2.45 Ghz = 1.6243499999999998e-24 eV
Gamma radiation at 3 x 10(19) Hz = 1.9889999999999998e-14 eV
So about 10(10) more energy per gamma ray photo or about 10 billion times more energy.