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V6: Advanced - Advanced Network (Section): SIP Session Timer (s)
V7/8: Advanced - SIP/RTP (Tab) - SIP(Section)__SIP Session Timer (s)
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If SIP Session Timer Support is enabled, this option specifies the SIP session timer in seconds. For instance, a Re-INVITE will be sent after 50% of its value has elapsed.
The detailed behavior of the Session timer is defined here: https://tools.ietf.org/html/rfc4028.
The Min-SE header sent in the SIP requests should also be taken into consideration because it defines the minimum accepted value for the Session Timer. The minimum value for Min-SE header recommended by the RFC is 90, which is as well sent by the Snom phones (the value of 90 cannot be changed). This limits the Session timer to a minimum value of 90.
Integer values, e.g. <0>, <2400>, <3200>.