To turn on Google Maps' voice navigation so it continues to play during a phone call, you need to adjust a specific setting within the app's navigation options. First, open the Google Maps app, then tap your Profile Picture or Initial in the top-right corner to access the menu, and from there select Settings, followed by Navigation settings (or just Navigation). Finally, scroll down within the navigation settings and look for the toggle switch labeled "Play voice during phone calls" and make sure to turn this option On; this will ensure that the turn-by-turn directions will play over the active call.