I guess you can't. See below
Restrictions
The following restrictions apply to the Intercom feature:
•
Intercom calls do not follow a coverage path.
•Hold—The system does not allow intercom calls to be placed on hold.
•Call Forwarding—Intercom calls cannot be forwarded.
•Transfer—The system does not allow an intercom call to be transferred.
•iDivert—The system does not allow an intercom call to be diverted.
•Call Pickup/Directed Call Pickup—The call pickup groups do not include intercom calls.
•DND—Intercom overrides Do Not Disturb (DND).
•If sufficient bandwidth does not exist, the intercom call fails.
•If two intercom calls are directed to a target, the first one goes through; the second fails with a busy tone.
•Barge and cBarge—Intercom does not work with Barge and cBarge.
•Conferencing—The system does not allow intercom calls to be conferenced.
•When an active call is being monitored or recorded, the user cannot receive nor place intercom calls
•Video is not supported with intercom.
