I'm not a SIP expert, nor to I want to become one (though I have had to learn a lot, including packet traces with Netmon 3). What I want to know is "under what conditions will the PBX get involved in relaying the audio?" I know it will if "PBX Delivers Audio" is set for an extension or a VOIP Provider. But I suspect that Replace and Re-Invite support for the VOIP provider affects the PBX's choice of audio delivery (direct or via PBX). I don't know what Replace or Re-Invite are so ... My reason for wanting to know this is that I have some softphones as extensions and they are capable of making direct SIP calls. It seems that the PBX doesn't get involved in these, or does it. All of my systems are on public IPs, no nasty "beast from hell" NAT routers. So this isn't a very clear request for help... and I apologize in advance. I just want to understand how the PBX treats the audio for calls from extensions addressed to SIP endpoints (which themselves may be extensions on other PBXs).