This event notifies the app that the session timer has expired
for a dialog, so the app should terminate the dialog by sending BYE.
The RA won't automatically send BYE, it is up to the application to
decide how to handle session expiration. However the RA will send a BYE
if this event is not processed by any SBBs. This is to ensure that
dialogs from deactivated services get cleaned up.