If you put your phone on top of a microwave whether the microwave is on or not then nothing will happen.
The microwave blocks radiation so the phone should be unaffected.
Even using your phone near your microwave is safe.
A cell phone makes more microwaves than any that leak out from a common microwave oven.
Your mobile will be that much closer to the microwave.
It's also safe to operate your laptop near your microwave oven.
Microwaves are radiation in a region of the electromagnetic spectrum, just like light and radio waves.
Your radio receiver detects radio waves when you tune your set to match the frequency of a particular station's broadcast.