Go to Call Routing > Intercom > Intercom Partition and create a new partition.
Go to Call Routing > Intercom > Intercom CSS and create a new CSS that containst the partition created above.
Go to Call Routing > Intercom >Intercom Directory Number and create 2 different directory numbers that we will use to communicate. Also associate the CSS created above, and choose a Default Device to which you will assign this particular number.
Go to the first phone we will configure under Device > Phone and modify the phone button template as shown below: It might be necessary to remove one of the options from the left square to make room for the new option; just select an unwanted option and clic on the arrow pointing down to the bottom square. Now, on the right square choose "Intercom  - Add a new Intercom", and click on the arrow pointing left to the left square. The option might appear on the left, at the bottom of the list.
Save the changes everywhere and reset the phones or apply configuration (depends on the Callmanager version).
Click on the Intercom option at the left side of the phone configuration page, this will take you to the Intercom Directory Number page.
Look for the "Speed Dial" field, and input the target number. This is the Intercom DN that will go off-hook in speaker mode when you press the Intercom number we are currently configuring. ***NOTE*** If you do fill the Speed Dial field, when pushing your Intercom DN button will open the line and prompt you to enter any Intercom number. Only an Intercom number will be accepted, any regular DN won't work and most likely will ring busy if it doesn't perfectly match an Intercom number you don't know about.
Do the same configuration to the other IP Phone assigning it the second Intercom DN we created.
With the correct CSS and partitions, and after filling the Speed Dial field on each device, we should now be able to press the button and go off-hook with the other device. You can choose if you want your Intercom line to go off-hook in headset mode or in speaker mode from the line configuration page.
The following models support the feature in both SCCP and SIP mode:
Callmanager 4.x and below (Document K62352022 applies to these versions)
Cisco CallManager does not have a dedicated intercom feature. However, you can use the Auto Answer feature in Cisco CallManager. Activatving this option or button causes the speaker phone to go off hook automatically when an incoming call is received.
To configure Auto Answer, perform these steps:
On the Cisco CallManager Admin page, go to Device > Phones > Select the Extension. Select Enable the Auto Answer feature.
Choose one of these options to activate the Auto Answer feature for the directory number:
Many sites with an existing PBX also have a paging system, allowing users to call an extension on the PBX that forwards the audio broadcast to overhead loudspeakers. This concept is useful in workshops, parking lots, and open plan areas where a called party is not near a telephone handset. PBX manufacturers may provide dedicated line cards that interface with external paging amplifiers.
The Cisco CallManager needs the Cisco 2610 router to be configured as an H.323 gateway device. The extension number for the paging port is defined under the Cisco CallManager Route Pattern configuration page, pointing to the Cisco 2610 H.323 gateway.
When the number for the paging system is dialed, a VoIP call is made between the IP handset to the E&M port on the gateway router. The voice port goes off hook. This is indicated by the E lead on pin 7 going from open circuit to closed circuit (with respect to the ground on pin 8). This off hook condition activates the pager system's control input, and the audio is sent on pins 4 and 5 of the voice port