You can set up to twelve numbers to ring when someone calls your line using the Simultaneous Ring feature. If any one of the lines answers a call (including your line), the other phones stop ringing. If no one answers the call, the caller returns to your office phone's voicemail.
After you've set the other lines to ring and the Simultaneous Ring options, you can turn the feature on and off using the Simultaneous Ring icon (a blue circle with a white phone) on the toolbar.
To turn on Simultaneous Ring: