[Freeswitch-users] Access to specific B leg headers on failed bridge

[David Witham]

I have a problem where an upstream provider will correctly send a SIP 486
Busy Here from their voice gateway with a Reason header containing Q.850
cause=17 when a B party is genuinely busy. However if we exceed certain
traffic thresholds, instead of receiving a SIP 503 with a retryable cause
code, their proxy sitting in front of their voice switch sends us a SIP 486
Busy Here and no reason header. Different user agent and also contains a
Server: header that the voice gateway responses do not. Therefore we start
failing calls that we would otherwise route advance to another upstream.

I want to be able to discern the difference between the two responses and
route advance on the false user busy. I've tried using variations of the
example LUA code found here
https://freeswitch.org/confluence/display/FREESWITCH/Cause+Code+Substitution+Example
but
what I have found is that, of the variables I seem to have access to at
this point in the call flow, sip_invite_failure_status=486
and last_bridge_hangup_cause=USER_BUSY are set the same in both cases. I
think I need to be able to check for the absence of a Reason header or
presence of the Server header to know that I have a false user busy.

My question is then, is it possible to obtain the specific headers that are
present in the B leg response after a failed bridge? I've tried addressing
sip_h*, sip_bye_h* type variables but none seem to be populated. I'm
guessing that I'm trying to address them in the A leg but they're only
present in the B leg. Any tips or advice would be greatly appreciated.

David
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