Hi Vinh,
Just to add a reference to the good info from Kelvin (+5 points for this Kelvin)
In the case of SRST a Virual Port to accomodate Dual Line is = to 1 DN :)
To set the maximum possible number of directories or virtual voice ports that can be supported by a router and to activate dual-line mode, octo-line mode, or both modes, use the max-dn command in call-manager-fallback configuration mode. To return to the default number of directories or virtual voice ports and to deactivate the dual-line mode or octo-line mode, use the no form of this command.
max-dn max-no-of-directories [dual-line | octo-line] [preference preference-order]
[number octo-line]
no max-dn
Syntax Description
Command Default
0 directory numbers, 1 channel per virtual port, 8 channels per virtual port, 0 preference, 8 numbers
max-no-of-directories
Maximum number of directory numbers (dns) or virtual voice ports supported by the router. The maximum possible number is platform-dependent; see the Cisco IOS command-line interface (CLI) help. The default is 0 directory numbers and 1 channel per virtual port.
dual-line
(Optional) Sets all Cisco Unified IP phones connected to a Cisco Unified SRST router to one virtual voice port with two channels.
octo-line
(Optional) Sets all Cisco Unified IP phones connected to a Cisco Unified SRST router to one virtual voice port with eight channels.
The max-dn command limits the number of Cisco Unified IP phone directory numbers or virtual voice ports available on the router.
The dual-line keyword facilitates call waiting, call transfer, and conference functions by allowing two calls to occur on one line simultaneously. In dual-line mode, all Cisco Unified IP phones on the Cisco Unified SRST router support two channels per virtual voice port.
The octo-line keyword facilitates call waiting, call transfer, and conference functions by allowing eight calls to occur on one line simultaneously. In octo-line mode, all Cisco Unified IP phones on the Cisco Unified SRST router support eight channels per virtual voice port.
http://www.cisco.com/en/US/docs/voice_ip_comm/cusrst/command/reference/srsa.pdf
Hope this helps!
Rob
