[Freeswitch-users] Understanding why FS replies with "407 Proxy Authentication Required" on internal INVITE

[Thilo-Alexander Ginkel]

Hello everyone,

we have a current FreeSWITCH installation (v1.10.6) that interfaces
internal users with upstream gateways. All clients work flawlessly, except
for an AVM FritzBox (short: FB), which exhibits the following behavior:

When FB initiates a call to another user or external number, FreeSWITCH
responds with a "407 Proxy Authentication Required" response although the
dialplan is the same as for the clients that do not exhibit this behavior.
This confuses the FB's SIP stack, which consequently reports a timeout and
drops the call.

A (redacted) SIP trace is available at:
https://gist.github.com/ginkel/5c7dd3723f92e0137d5f357451dc7ac6

All clients as well the FS server reside in the same RFC1918 network.

Other Polycom or Yealink clients are unaffected.

Any ideas what might cause this?

Thanks,
Thilo
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