This info can be found in the Configuration and Administration Guide for Cisco Unified Customer Voice Portal Release 8.0(1) page 356
Session Timer RFC 4028 support in Unified CVP. Unified CVP, acting as a Back-to-Back User Agent (b2bua) will transparently pass thru the headers for Supported, Session Expires, and Min-SE on the call legs. However, since the IOS gateway does not set a Session Expires header by itself, it will not request session refreshing. By default, Unified CVP adds a Session-Expires: 1800;refresher=uac value to the 200OK response on the initial invite sent into Unified CVP by the caller, if the Session Expires header is not already present in the 200OK of the outbound leg. This value will cause the gateway, as UAC, to send the refesher invites in mid call every 1800 seconds. This configuration allows the gateway to clean up a zombie call, due to a failed refresher reinvite, if the Unified Call Server is taken down in mid call. The setting on Unified CVP to disable adding a Session Expires header is configurable.
In plain English: The Sip subsystem runs a check to make sure there are no stale calls ever left up. These types of calls can tie up resources. This feature then tears down calls where there are no keepalive response receieved during a given interval. If you are seeing this and it is a legitimate call being torn down, (there are siuations while playing a prompt there may not be a repsonse for the keep alive interval) the call could be mistaken for a zombie call. In these instances you can change the timer and increase it just above the longest anticipated call duration. That will typically address that outlier example. The setting can be found below.
in $CVP_HOME/conf/sip.properties. Change/Add SIP.ZombieCheckCallLength=180 (180 sec. to start)
