Session-Expires

  From the Session-Expires header field in the response, both UAs know

   that a session timer is active, when it will expire, and who is
   refreshing.  At some point before the expiration, the currently
   active refresher generates a session refresh request, which is a
   re-INVITE or UPDATE [3] request.  If the refresher never gets a
   response to that session refresh request, it sends a BYE to terminate
   the session.  Similarly, if the other side never gets the session
   refresh request before the session expires, it sends a BYE.

https://www.ietf.org/rfc/rfc4028.html