Creating an Intercom
Cisco CME supports single-button push-to-talk and push-to-respond intercom lines. You can create an intercom arrangement between any two (multiline) IP phones that support speakerphone operation. You can even operate an intercom across a VoIP connection using either SIP or H.323. Cisco CME's intercom function is built using two functions:
- Autodial at the initiating end of the intercom
- Autoanswer-with-mute at the receiving end
To create an intercom you assign a line button on each of the two phones to operate as an intercom line. Pressing the intercom line button selects the line and triggers the autodial function toward the second phone. The receiving phone receives the incoming intercom call on its intercom line. This line autoanswers the call and activates the phone in speakerphone mode and sounds a beep. It also forces the speakerphone to mute to protect the privacy of the intercom recipient. The audio path is open from the initiator to the receiver. To respond to the intercom, the recipient simply presses the mute button to unmute the audio path back to the originator.
The intercom Command
Example 5-21 shows a configuration of an intercom between two IP phones.
Example 5-21 Intercom Lines
router#show running-config ephone-dn 1 dual-line number 1001 name John Smith ephone-dn 2 dual-line number 1002 name Jane Smith ephone-dn 3 number 1111 intercom 1112 label Jane ephone-dn 4 number 1112 intercom 1111 label John ephone 12 mac-address 000d.1234.0efc button 1:1 2:3 ephone 15 mac-address 000d.5678.0dcf button 1:2 2:4
Example 5-21 shows two phones. John's phone has button 1 as his primary extension line. Button 2 on John's phone is an intercom line. This line is set to autodial Jane's phone using the number 1111. The button is labeled "Jane" to show that pressing the button intercoms to Jane.
Jane's phone is configured to match to John's phone. Button 1 on Jane's phone is her primary line. Button 2 is set to autodial John's phone and shows the label "John" next to the button. The default configuration for an intercom line is that it autoanswers with mute for any incoming call. Autoanswer can be disabled if desired using the no-auto-answer command option.
Example 5-21 shows a fully symmetric two-way intercom arrangement. John can intercom to Jane, and Jane can intercom to John.
Individual phones may have more than one intercom. The maximum number of intercoms per phone is limited only by the number of available buttons. In general, using intercoms on single-line phones such as the Cisco 7910, 7905, and 7912 is not recommended. These phones do not have a built-in (hands-free) microphone and, therefore, cannot be unmuted without lifting the phone's handset.
In the previous example you saw that an intercom line autoanswers any incoming call. The incoming intercom does not perform any type of cross-check on the calling party to ensure that it matches the outgoing intercom destination. This arrangement lets you create a many-to-one intercom, which is useful when you have a single shared assistant working for multiple executives, as shown in Example 5-22.
Example 5-22 Shared-Line Intercom
router#show running-config ephone-dn 1 dual-line number 2101 name Executive1 ephone-dn 2 dual-line number 2102 name Executive2 ephone-dn 3 dual-line number 2103 name Executive3 ephone-dn 4 dual-line number 2201 name Assistant ephone-dn 5 number 1110 intercom 1110 label Intercom ephone-dn 6 number 1111 intercom 1110 label Intercom ephone-dn 7 number 1112 intercom 1110 label Intercom ephone-dn 8 number 1113 intercom 1110 label Intercom ephone 12 mac-address 000d.1234.0efc button 1:1 2:6 ephone 13 mac-address 000d.5678.0dcf button 1:2 2:7 ephone 14 mac-address 000d.4321.0ef7 button 1:3 2:8 ephone 15 mac-address 000d.4132.f7e4 button 1:4 2:5
In Example 5-22, ephones 12 to 14 belong to the three executives 2101 to 2103. The fourth phone, ephone 15, belongs to the assistant. Each of the four phones has its primary line associated with button 1. The second buttons on the three executive phones are configured to intercom to the assistant's intercom line. This configuration provides a many-to-one intercom. Any of the executives can press the button 2 intercom on his or her phone to talk to the assistant. Of course, only one intercom conversation can exist at one time.
Note that the assistant's intercom (ephone-dn 5) is set to autodial itself. It is not possible to create a one-to-many intercom path. The assistant's phone could also be configured to autodial one of the executives.
In the previous many-to-one intercom example, you saw that the assistant's intercom is configured as a one-way or receive-only intercom. This is done by configuring the intercom line to autodial itself. When the assistant presses the intercom button, he or she hears busy tone.
You can use this technique to configure a one-way intercom even in a simple one-to-one intercom arrangement. A better configuration for a one-way, one-to-one intercom is to use the noautoanswer command option, as shown in Example 5-23.
Example 5-23 One-to-One Intercom
router#show running-config ephone-dn 1 dual-line number 2101 name Executive ephone-dn 4 dual-line number 2201 name Assistant ephone-dn 5 number 1110 intercom 1111 label Intercom ephone-dn 6 number 1111 intercom 1110 label Intercom no-auto-answer ephone 12 mac-address 000d.1234.0efc button 1:1 2:6 ephone 15 mac-address 000d.4132.f7e4 button 1:4 2:5
With this arrangement, you still have a direct one-to-one dedicated line between the executive and the assistant. Only the assistant's phone autoanswers incoming calls presented to its intercom line. If the assistant presses the intercom line, the phone still autodials the executive's intercom line. The executive's phone simply rings as for a normal call.
Dialable and Private Intercoms
All the previous intercom examples in this chapter show intercoms configured with ordinary extension numbers. This means that you can dial into the intercom lines from any phone, not just the phones configured for intercom. You can even put the intercom numbers into the Cisco CME system directory or speed-dial lists.
You can make the intercom more restrictive by using the extended DTMF digits A–D as part of the intercom phone number. Because IP phones do not have the A–D DTMF digit keys available on their keypads, if the phone number includes A–D digits, it cannot be dialed from an ordinary phone.
Example 5-24 shows how to restrict intercom numbers so that they cannot be dialed from an ordinary telephone.
Example 5-24 Nondialable Intercom
router#show running-config ephone-dn 1 dual-line number 2101 name Executive ephone-dn 4 dual-line number 2201 name Assistant ephone-dn 5 number A110 intercom A111 label Intercom ephone-dn 6 number A111 intercom A110 label Intercom ephone 12 mac-address 000d.1234.0efc button 1:1 2:6 ephone 15 mac-address 000d.4132.f7e4 button 1:4 2:5
In this example, you see that the intercom numbers are set to A110 and A111 using the DTMF A digit as the first digit. This technique is also useful if you are using a dial plan that has only two- or three-digit phone numbers. Use of the extra A–D digits for intercom can free up space in the normal number range for use on regular extension numbers.
A courtesy phone is a phone in a publicly accessible area. The phone is allowed to make calls only to internal numbers, such as calls from a lobby, to request assistance. The phone is not allowed to make calls directly to other extensions or to make external calls.
You can create a courtesy phone using the Cisco CME intercom feature. If you configure a multiline phone with normal extensions and intercom extensions, the phone automatically selects the first available normal extension when you lift the phone handset. The phone never auto selects the intercom line, even if all available normal extension lines are in use (for example, by shared phones).
You can configure an IP phone with only a single line where that single line is an intercom. In this case, when you lift the handset off-hook, the only possible line selection is the intercom line. This line is selected, and the phone autodials to the configured intercom destination. You can use this configuration technique to create a courtesy phone that always dials a fixed destination when the phone is taken off-hook.