When should I use email instead of a phone call?

Use email when you need a record, when the details are complex, or when you want to respect the recipient's time.

A phone call interrupts the other person. It forces them to stop their work and switch context. An email is asynchronous. It arrives as a notification instead of an interruption and allows the recipient to respond thoughtfully when ready.

It is the difference between shouting across the deck and leaving a written log entry for review at dawn.