I've grepped in the code for "407",
"proxy_challenge",
challenge calls,
but none seems to be possibly called in my case. Although I don't know
much about openser internal code and I'm still not able to find where
the "best response selection" is made.
I would seriously recommend logging all decisions you take in the config
file using x_log(), so you can follow a transaction in the logfile.
By default the logfile is filled with mostly what OpenSER is doing, while
the "why" is much, much more interesting.
--
Andreas Sikkema