Designing and Implementing an IP Paging System – How to Intercom with VoIP (4 of 4)

March 24, 2011 by Garrett Smith

Note: This is the 4th and final installment of a four part series detailing the design and implementation of an IP paging (paging over VoIP) system:  Part 1, Part 2, and Part 3.

What is an intercom call?

Intercom differs from paging because in an intercom call, a single user will intercom another user or endpoint and essentially setup a point-to-point SIP call only, when the recipient gets an intercom call the phone does not ring but rather goes off-hook to speaker and audio is transmitted between the two parties. A single beep tone can also be configured to play to the user receiving the intercom before audio is transmitted to let them know that they are being alerted via intercom.

In today’s VoIP world, most IP PBX’s have the ability to setup intercom between two endpoints. This can be achieved in a few ways.

Intercom Calling – Method #1

The first method is to configure an intercom prefix. Most solutions that are Asterisk based already have this prefix setup, but you can change it if need be. After an administrator determines the prefix users can simply dial that intercom prefix, then dial the SIP extension they wish intercom and press send from their IP Phone.

Let’s say for instance our dialing prefix for intercom was #80 and Chris (me) wanted to intercom Nate who’s SIP extension is 300, all I would need to do is dial #80300 then press  send from my IP phone and Nate’s will phone will beep, then immediately go off-hook to speakerphone and transmit the audio between us.

Keep in mind with this scenario that, most IP phones must be configured to receive intercom calls and also go off hook automatically.

Here is a good example of how this would look:

Intercom Prefix

Intercom Calling – Method #2

The second method of intercom calling is to use the same intercom prefix as noted before but this time, our intercom device will not be a desk phone but either an indoor or outdoor call button with intercom functionality.

For example, the Cyberdata VoIP Indoor Intercom is a door entry device with a ringer button and Valcom offers a SIP door intercom.

These devices are configured very much like a SIP phone as they are merely another SIP extension on the VoIP PBX. They are connected to the IP network via cat5 and most support standard POE to simplify wiring to the end user or installer.

Once the intercom is properly registered to the IP PBX via its web GUI configuration, the administrator will then set a specific SIP extension for this device to automatically intercom. In most cases, these can be users such as the front desk in a hotel or school, a security office, or anyone permitting users inside a building.

Residential Intercoms

We have also seen a huge uptick in residential users and apartment style setups use these devices in their deployments. After these are setup, users engaging with the intercom device will simply press the CALL button (no dialing is necessary) and the intercom will automatically intercom the SIP extension is configured to dial and establish two-way audio between the two users.

For instance, Chris is an employee at VoIPSupply and wishes to access the building afterhours where the Security watchman Nate is working. Chris presses the outdoor intercom on the outside of the building and immediately starts communicating with Nate at his SIP extension, 300.

Nate recognizes that it is Chris and then sends a DTMF signal back to the intercom device to engage with a door strike which unlocks the door for Chris.

Here is a good look at how this would work:

Intercom Door Access 

Using Same DTMF Method Across All Devices

The ** (listed in the image above) prefix can also be configured on the Intercom unit via its web GUI (graphical user interface) configuration. The only requirement is that the same DTMF (dual-tone multi-frequency signaling) method is being used for:

  • The Intercom Unit
  • The IP PBX SIP Extension for the Intercom Unit
  • And the IP Phone communicating with the both the IP PBX and the Intercom

For instance, if an admin chooses to use the DTMF method RFC2833, then the Intercom, SIP extension, and the IP Phone must also use the RFC2833 DTMF method.

What Happens if the Same DTMF Method is Not Used?

If one of these devices is not using the same DTMF method as the others, for example say, the IP phone is set to use Inband or SIP INFO DTMF method type then when the user dials ** to engage the door strike neither the IP PBX nor the Intercom will know what to do since they are “talking” on the RFC2833 DTMF method.

The easiest way to put it is this:  If RFC2833 speaks English and Inband speaks Chinese then in most cases these two parties will not be able to understand one another. 


I hope this post better describes how to intercom using VoIP.  If you have any questions on these solutions, please feel free to contact our sales department for more information.  This concludes our 4 part segment on IP paging systems design and implementation.

Further Reading


  • John Orge

    Our office has an intercom system that looks similar to this. The way I understand it you pick up the phone push the intercom system to page the whole office(like a supermarket) is there a way to call the intercom from a cell phone so I would be able to page from anywhere in the office without having to find an internal phone?

    • Hi John,

      Thank you for your comment! On your cell phone, you can install a softphone to interact with the paging group or extensions. If you have more questions, you are welcome to submit a ticket here:

      Thank you,

  • What is the rule of thumb to estimate the required budget for a paging system?

    • Hi Danilo,

      Thanks for reaching out to us. Our VoIP consultant, Joe Shanahan, will be helping you answer your question shortly. Here I attach his contact information for your convenience.

      Joe Shanahan
      [email protected]

      Thank you!

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