Marcello,
I did a little research on the cisco "outbound sip proxy" . And I don't believe it is applicable. According to the documentation this command is used when using SIP endpoints (ip phones) with a CME router. This is the default mode, and on inbound calls to a SIP endpoint would cause to hairpin back out, thus causing the call to fail. So if one is using SIP endoints one might want to disable this feature. My IP phones are using SCCP, thus it doesn't apply.
But, what the hell, I did try it any ways. So I configured it globally and it had not effect on the SDP contact information being rewritten to a public ip address. And thus did not change the symptoms in any way. It is my understanding that it is the responsibility of the sip proxy daemon (sipproxd) plug-in running in pfSense to rewrite the SDP contact address information from the private to the public address. Is seams that this plugin is just not working.
Is there a way to verify or view the activity of the siproxd service. I have restarted it. But beyond that what can I do? What about debugs on the PFSense server? I'm sure there is something, just not familiar with BSD.
Brian