The only documentation available is the following one
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cuipph/8811_8841_8851_8861/10_5/english/adminguide/P881_BK_A92A3B94_00_adminguide-8811_41_51_61-10_5/P881_BK_A92A3B94_00_adminguide-8811_41_51_61-10_5_chapter_01010.html#CUIP_TK_SEB6C6C7_00
which leads to
http://www.cisco.com/c/en/us/support/docs/voice-unified-communications/unified-communications-manager-callmanager/42573-idle-url.html
Have you tried it yet to check if it works?
Manish