I had the same question/issue myself, and would be interested to know if there is a way. For myself, I had to do two things to get this to work:
1. Disable authentication so that FS would not itself generate a challenge. In vars.xml I added this:
<X-PRE-PROCESS cmd="set" data="internal_auth_calls=false"/>
and in my internal.xml (because calls were coming in on my internal profile):
<param name="accept-blind-auth" value="true"/>
2. Then I had to hack mod_sofia to create a new channel variable containing the received Proxy-Authorization on the inbound leg, and also let me set it on the outbound leg.
After that it worked, but I would still prefer a solution that didn't require changing mod_sofia, if one exists...
>>Hi All!
>>
>>I would like to know if there is a param we could set in sofia profile to let freeswitch relay Proxy-Authentication requests to the caller instead of absorbing it.
>>
>>Regards,
>>Hamid