Description: 
Configures the SIP session timer of T4 (in seconds). 
T4 represents the maximum duration a message will remain in the network. 
Web User Interface: 
Settings->SIP->SIP Session Timer T4 (2.5~60s) 
Phone User Interface: 
None 
To configure session timer via web user interface: 
1.  Click on Settings->SIP. 
2.  Enter the desired value in the SIP Session Timer T1 (0.5~10s) field. 
3.  Enter the desired value in the SIP Session Timer T2 (2~40s) field. 
4.  Enter the desired value in the SIP Session Timer T4 (2.5~60s) field. 
 
5.  Click Confirm to accept the change. 
Session Timer 
Session timer allows a periodic refresh of SIP sessions through an UPDATE request, to 
determine whether a SIP session is still active. Session timer is specified in RFC 4028. IP phones 
support two refresher modes: UAC and UAS. Whether the endpoint functions as a UAC or a 
UAS depends on the UA that initiates the SIP request. If the initiator is configured as UAC, the 
other client or the SIP server will function as a UAS. If the initiator is configured as UAS, the 
other client or the SIP server will function as a UAC. The session expiration is negotiated via the 
Session-Expires header in the INVITE message. The negotiated refresher is always the UAC and