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.