No, it is not safe to use a microwave if you accidentally nuked your phone inside, even if the phone turns completely off. Microwaving a phone (or any electronic device) can cause severe damage to the phone's internal components and potentially create safety hazards.
When you put a phone in a microwave, the metal components in the phone, such as the battery, circuitry, and other internal parts, can react to the microwaves and generate electrical currents. This can lead to short circuits, overheating, and even fires inside the microwave.
Microwaves are designed to heat food by emitting electromagnetic waves that specifically target water, fat, and sugar molecules, causing them to vibrate and generate heat. The metal components in electronic devices, including phones, can reflect these microwaves and cause arcing, which is the production of sparks. This arcing can damage the microwave's interior and, in some cases, result in a fire.
If you accidentally nuked your phone inside a microwave, it's crucial to avoid using the microwave until it has been inspected and, if necessary, repaired by a qualified technician. Additionally, if you suspect any damage to the phone, do not attempt to turn it back on or charge it, as it may pose a risk of fire or electrical shock.
Always exercise caution when using household appliances and never put electronic devices inside a microwave or any other appliance not intended for them.