Configuring a basic shared line in CME is easy. Simply assign the same ephone-dn to multiple ephones, as shown in Example
Configuring a Basic Shared-Line System
CME_Voice(config)# ephone-dn 10 dual-line
CME_Voice(config-ephone-dn)# number 1010
CME_Voice(config)# ephone 8
CME_Voice(config-ephone)# button 1:10
CME_Voice(config-ephone)# restart
CME_Voice(config-ephone)# exit
CME_Voice(config)# ephone 9
CME_Voice(config-ephone)# button 1:10
CME_Voice(config-ephone)# restart
CME_Voice(config-ephone)# exit
Once the phones restart, the same DN (1010) appears on both. Incoming calls to DN 1010 will ring on both phones, and whoever answers the call first gets the call. The main problem with this configuration is that only one person can use the shared line at a time. If the line is in use, it appears on other phones using the same line,Other environments may desire the shared line be made available to others (to place and receive calls) even if the shared line is in use. For example, take a technical support group of five employees who all receive support calls from the organization on the shared DN 1010. Just because one person from the support group is on a call should not mean that the others cannot make or receive calls. This situation calls for multiple ephone-dns with the same extension number. Example 5.14 shows the configuration.
Configuring a Shared-Line System with Multiple Ephone-DNs
CME_Voice(config)# ephone-dn 10 dual-line
CME_Voice(config-ephone-dn)# number 1010
CME_Voice(config)# ephone-dn 11 dual-line
CME_Voice(config-ephone-dn)# number 1010
CME_Voice(config)# ephone 8
CME_Voice(config-ephone)# button 1:10
CME_Voice(config-ephone)# restart
CME_Voice(config-ephone)# exit
CME_Voice(config)# ephone 9
CME_Voice(config-ephone)# button 1:11
CME_Voice(config-ephone)# restart
CME_Voice(config-ephone)# exit
The configuration in Example 5.14 takes care of the outgoing call issue because the two ephones are using truly unique, dual-line ephone-dns. If ephone 8 picks up the line, the CME system only sees ephone-dn 10 in use and does not show the line-in-use indicator on ephone 9. Incoming calls do pose a problem. When the CME router receives a call for 1010, it now sees two equal cost paths to get there (ephone-dn 10 and ephone-dn 11). So how is it going to handle this dilemma? Just pick one completely at random. So, sometimes ephone 8 will get the call and sometimes ephone 9 will get the call. If this is your desired behavior, great! You are done with the configuration. However, most people would like more control over how the call flows than a random line selection. You can achieve control over which ephone receives the call by using the preference and huntstop commands. The preference command dictates which ephone-dn is more preferred than the other by assigning a value from 0 to 10, where the lower preference numbers are better (that doesn't make much sense, does it?). For example, if you were to modify the ephone-dn configuration from Example 5.14 to look like Example 5.15, ephone-dn 10 would always receive incoming calls for the DN 1010. Only if the ephonedn 10 was busy or unavailable would ephone-dn 11 begin receiving incoming calls.
Using the preference Command with Multiple Ephone-DNs
CME_Voice(config)# ephone-dn 10 dual-line
CME_Voice(config-ephone-dn)# number 1010
CME_Voice(config-ephone-dn)# preference 0
CME_Voice(config)# ephone-dn 11 dual-line
CME_Voice(config-ephone-dn)# number 1010
CME_Voice(config-ephone-dn)# preference 1
That brings up another potential problem. Each of the ephone-dns are configured with the dual-line syntax. This enables them to place calls on hold and make an additional call, transfer a call, start a conference call, or receive calls via call waiting. Whoa, put the brakes on right there. Because ephone-dn 10 was configured as a lower preference than ephone-dn 11, the first call to DN 1010 would go to ephone-dn 10. If a second call came in to DN 1010 while ephone-dn 10 was still on the active call, the call would be received via call waiting on ephone-dn 10 rather than rolling over to ephone-dn 11. So, you would have one employee (on ephone-dn 10) taking on two calls while the other employee (on ephone-dn 11) sits idle. That doesn't seem efficient, does it? This is where the huntstop command comes in handy. The huntstop command has two forms: huntstop and huntstop channel. The name of this command describes its function. It stops the CME router from hunting for other matches to the destination pattern. Simply entering huntstop tells the CME router, 'stop hunting for other matches with this ephone-dn.' Entering huntstop channel says to the router, 'stop hunting for other matches with one channel of this ephone-dn.
Using the preference Command with huntstop
CME_Voice(config)# ephone-dn 10 dual-line
CME_Voice(config-ephone-dn)# number 1010
CME_Voice(config-ephone-dn)# preference 0
CME_Voice(config-ephone-dn)# huntstop channel
CME_Voice(config-ephone-dn)# no huntstop
CME_Voice(config)# ephone-dn 11 dual-line
CME_Voice(config-ephone-dn)# number 1010
CME_Voice(config-ephone-dn)# huntstop channel
CME_Voice(config-ephone-dn)# preference 1
Notice the configuration of ephone-dn 10 uses the huntstop channel command followed by the no huntstop command. This may seem counterintuitive at first, but follow the logic: if you use only the huntstop channel command, the router will simply stop hunting if the first channel of ephone-dn 10 is in use. Additional calls to DN 1010 will result in a busy signal. By combining the huntstop channel command with the no huntstop command, you are telling the CME router, 'Stop hunting for other channels on ephonedn 10, but don't stop hunting completely! Look for other ephone-dns with this DN.' This results in ephone-dn 11 receiving a call to DN 1010 if ephone-dn 10 is currently active on a call. If both ephone-dns 10 and 11 are on an active call, an incoming call to DN 1010 will receive a busy signal. Chapter 5: Basic CME IP Phone Configuration 151.
Now back to the original scenario: There are five employees in an organization's technical support group who receive calls on DN 1010. The configuration in Example 5.16 will accomplish this objective (if there were an additional three ephone-dns added with DN 1010 for the other employees), but the tech support employees who have the preference 0 and 1 ephone-dns assigned to their IP phone will get lots of calls while the tech support employees with the preference 2, 3, and 4 ephone-dns will only get calls if the first two are busy. A better system might have all the IP phones ring when on a call to DN 1010, and whichever tech support employee answers the call first will take the call.This design is a custom fit for ephone-dn button overlays. Overlaying ephone-dns allows you to assign multiple DNs to a single extension. Back in Table 5.2, there were three 'overlay separators' listed that you can use with the button command. To save page flipping,here they are:
o separator: Overlay line with no call waiting
n c separator: Overlay line with call waiting
n x separator: Overlay line with line rollover to other lines on Cisco IP phone.
Configuration using Button Overlays
CME_Voice(config)# ephone-dn 10
CME_Voice(config-ephone-dn)# number 1010
CME_Voice(config-ephone-dn)# preference 0
CME_Voice(config-ephone-dn)# no huntstop
CME_Voice(config)# ephone-dn 11
CME_Voice(config-ephone-dn)# number 1010
CME_Voice(config-ephone-dn)# preference 1
CME_Voice(config-ephone-dn)# exit
CME_Voice(config)# ephone 8
CME_Voice(config-ephone)# button 1o10,11
CME_Voice(config-ephone)# exit
CME_Voice(config)# ephone 9
CME_Voice(config-ephone)# button 1o10,11
CME_Voice(config-ephone)# exit
CME_Voice(config)# telephony-service
CME_Voice(config-telephony)# restart all
An incoming call to DN 1010 would now ring both ephones 8 and 9 because ephone-dns 10 and 11 are overlayed on button 1. Notice that Example 5.17 uses the o separator, which means overlay with no call waiting. The first call that comes in to DN 1010 will cause both ephones 8 and 9 to ring. Whichever phone answers the call will take responsibility for it and the CME router will flag the ephone as busy' (in a call). If a second call comes in while the first call is still active, the ephone that did not answer the first call will ring. The ephone that is still active on the first call will not hear a call waiting beep while the line is ringing. If you wanted to have call waiting active for the overlay lines, you could change the button command syntax to button 1c10,11 for both phones. If you have changed to a c button separator, the phone currently on an active call would hear a call waiting beep. This is not the call waiting beep of ephone-dn 10 (which received the first call) but the call waiting beep of ephone-dn 11 (which is receiving the second call).
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