You can intercom another phone in the office by dialing an 8 before dialing their extension number. They will hear a distinct beep and then your voice over the speakerphone. They can answer back hands-free. If they are on active call when you try to intercom them, they can answer, reject, or ignore the incoming announcement.
Dial 8+Ext number and press the soft ‘Dial' key found at the bottom of the display screen
Make your announcement using the handset or the speaker key on the phone
The extension dialed will hear a notification beep and then your voice. If that extension is on an active call, the phone will treat the intercom as another incoming call that can be answered, rejected, or ignored.