Cisco 7900 Series - Manual (Page 10)

Step 2 : After you have selected Rapid Broadcast, scroll down
or select Option 5. (Corvero Intercom) Type in the extension
of the phone you are trying to reach and hit the connect
button. After the phone prompts you that your recipient has
been contacted, speak into the handset with a very clear voice.
**You can use speakerphone with Intercom but not All –
Call.**