This is a hard-coded limitation of CCX and cannot be changed. The Solution Reference Network Design for Cisco Unified CCX and Cisco Unified IP IVR, Release 8.0 covers this in the Multiple Lines Support section.
Cisco Unified CCX supports using the 6900/8900/9900 series phone as agent devices. Cisco Unified CCX monitors calls on multiple lines in order to support Join Across Line (JAL) and Direct Transfer Across Line (DTAL) on those agent devices. Up to 4 lines are monitored by Cisco Unified CCX, these include 1 ACD line and 3 non-ACD lines but only the ACD line can be controlled from the agent desktop. If more than four lines are configured on an agent device, the agent cannot login using that phone. JAL and DTAL are also supported on agent devices using 7900 series phone and up to 4 lines can be configured on those phones.
Note For agent devices with monitored non-ACD lines, make sure to inlcude the non-ACD lines as the CTI controlled lines when performing sizing for Cisco Unified CM server(s).
