Is fire more harmful than a phone? I have read somewhere that the higher the frequency of electromagnetic radiation, the higher the damage it causes to your body, and visible light has a very high frequency in comparison to microwave rays, released by phones.
So, does this mean that fire is, comparatively, more harmful than phone?
Visible light has a frequency from about 400-700 THz and microwaves have a frequency from about 300 GHz to 300 MHz.
Also, does this mean that the higher the energy, the higher the damage (as frequency is directly proportional to total energy)?
 A: You cannot simplify the effects of EM radiation on biological systems to simply $E=hf$ because  different materials absorb or transmit different frequencies preferentially. $E=hf$ tells us the energy per photon, but it doesn't tell us how much is absorbed by any particular type of cell. It also doesn't tell us the intensity of the radiation (energy per second per area). 
Regarding absorption preferences, x-rays are absorbed by bone but not so much by vascular tissue.  Different frequencies (wavelengths) of x-rays are absorbed in different amounts.  Lower frequency radio waves and microwaves (MHz to GHz range) are absorbed by water-bearing tissues, but the energy/photon is much less than an x-ray photon.
Visible light is mostly harmless to skin, but slightly higher energy UV radiation will damage skin DNA due to a resonance absorption in the DNA.
Gamma rays (E/photon ~> 100 keV ) definitely have a greater potential to damage cells if they are absorbed, but again if the intensity is low, there is generally negligible damage to the system.
So, high intensity microwave/radar could possibly be more dangerous than low intensity gamma, depending on the tissue involved.  It's a very complex problem that requires good experimental research.
Bottom line:  absorption coefficients/resonances and source intensity are more important than energy/photon.
