Yes, even if not on the same frequency spectrum, depends on the microwave and how is made, but still, it may allow signal to go in (your phone could ring), but it should block reverse emissions

Better thing if you have time and materials, wrap the phone in plastic foil or a bag or something, take the aluminium paper roll or something that does isolate, and wrap it over the plastic, repeat as much as you want, 3-4 layers should be safe, leave the last wrap as plastic so that insulates, and put it in the microwave (or if you have a metal trash can, you can put a plastic bag inside with the content (ex wrapped phones, pc, etc), and close it, should add as an additional layer of insulation ( but be sure the metal never touches things inside and plastic is always the last layer you use)