Microwaving a cell phone for any length of time can cause irreparable damage to the device through electrical arcing and scorching. The electrical discharge will damage or destroy the delicate circuitry inside the phone and may even cause the battery to rupture or start a small fire.
Metals react to microwaves by reflecting the energy in various ways. Flat sheets, such as the aluminum panels used to crisp foods like pizza or pocket sandwiches, simply heat up and reflect extra energy into the food. More complex structures such as silverware or the circuits inside cell phones react in more violent ways. The microwave energy reflects back and forth between various parts of the metallic structure, creating sparks and electric arcs. This causes the metal to heat up rapidly and may melt surrounding plastic or scorch other materials. The reflected microwave energy may also damage the magnetron, rendering the microwave inoperative.
In 2014, a fake ad circulated on the Internet touting a new feature of the iPhone that would allow users to charge it using the microwave. While this was not the first time this prank appeared online, it received considerable coverage in the news over concerns that multiple people had fallen for the prank and destroyed their cell phones.