Using Private Lines

Similar in some ways to the courtesy phone application for the intercom command, Cisco CME 3.2 introduces the FXO trunk command. This can be used to provide an emulation of a dedicated private PSTN line for a specific phone user. This allows Cisco CME to create the user appearance that one of the buttons on a Cisco IP phone is directly connected to a specific PSTN subscriber line (usually a dedicated FXO port connected to a specific PSTN phone number). One potential application for this is in a bank branch office where the bank manager has an internal extension number for regular calls (perhaps forwarded to the manager by a receptionist) plus a direct private line used for important clients. The private line also has voice mail service provided by the PSTN. This provides message waiting indication (MWI) by way of a stutter dial tone. To hear the MWI indication, the manager selects the private line and hears dial tone provided by the PSTN.

Example 5-25 shows how the trunk command is used to create a private line on an IP phone.

Example 5-25. FXO trunk Command

router#show running-config
voice-port 1/0/0
 connection plar-opx 1082 
dial-peer voice 82 pots
 destination-pattern 82 
 port 1/0/0
ephone-dn 10
 number 1010
 name manager
ephone-dn 11
 number 1082 
 name private-line
 trunk 82 
ephone 1
 button 1:10 2:11

Example 5-25 shows ephone 1 configured with two lines. Button 1 is a normal extension (number 1010) using ephone-dn 10. Button 2 is the private line using ephone-dn 11. Incoming PSTN calls on FXO port 1/0/0 are routed directly to ephone 11 (extension number 1082) by the connection plar-opx 1082 command that is shown under voice-port 1/0/0.

Outgoing calls on the private line are routed to voice port 1/0/0 by the trunk 82 command within ephone-dn 11. This causes all calls dialed on ephone-dn 11 to have the digits 82 prefixed to the dialed number. The 82 prefix causes the calls to match to the destination-pattern 82 in dial peer 82 and, thus selects voice port 1/0/0 for the outbound call.

When the phone user dials the number, such as 555-0510, the trunk command prefixes the digits 82 to create the number 825550510. The leading digits 82 match the destination pattern for the voice port dial peer. The leading 82 digits are stripped off, and the remaining digits, 5550510, are forwarded to the PSTN line. Note that in some cases, you may need to adjust the time delay before the digits are passed to the PSTN line to avoid their being sent before the PSTN line is ready to accept them. You can do this using the prefix command (under the POTS dial peer) and using commas to insert 1-second units of delay. For example, the command prefix,, inserts 2 seconds of delay.

The appearance that the phone button is directly connected to the FXO port's PSTN line is obtained by means of the one-to-one association that's created by the combination of the connection plar-opx binding of the FXO port to the ephone-dn, plus the trunk binding of the ephone-dn back to the FXO port. Because of this arrangement, whenever the FXO port is in use, it follows that the ephone-dn is also in use. Therefore, the ephone-dn's status reflects the FXO port's status.

To maintain this direct one-to-one binding, call operations that could break the one-to-one binding are disabled by the trunk command. This means that functions such as call transfer and call forwarding are not supported when the trunk configuration is used. However, functions such as call hold and conference are supported, including the ability to join two ephone-dn trunk lines in a three-party conference.

You can use the trunk command and connection plar-opx command independent of each other. For example, you can create other private line-like call behaviors where incoming calls are directly routed to a specific extension using the connection plar-opx command but outgoing calls use the common "Dial 9 for an outside line" approach.

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