On Android, open the Google Assistant app, then tap the compass icon in the lower right. Press on your avatar (top left), then choose Settings, Assistant, and Phone. Next, toggle the Access with Voice Match button to "on" and you'll be able to access Google Assistant from the lock screen. If you haven't already trained the app to recognize your voice, you'll need to go through a short setup process for that, too. Once you've laid the groundwork, just say, "Hey Google" or, "Okay Google," on the lock screen to access your assistant.