The Phishing Report Button is available on various platforms. If you're not using Outlook on desktop, scroll down and open one of the other options.

On desktop (Windows, macOS)
On desktop, the Phishing Report Button is part of the normal Outlook toolbar.

On iPhone 
On Android 
In the Outlook Web App (OWA)
With the Outlook Web App, you must first click on the overflow icon and then click the Phishing Report Button.