Regarding the value 4 -- that is an example, it can be any value from 0 to 31 -- the range
of flags that can be used inside kamailio.cfg.
The parameter is mandatory from old times, when tracking dialogs was done by setting the
respective flag for the invite transaction using setflag(N). Now there is the alternative
(recommended) of using dlg_manage(), so I pushed a patch to master branch to remove the
check of dlg_flag being not set and start without it.
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https://github.com/kamailio/kamailio/issues/372#issuecomment-149236845