We have our ISP set up QoS using DSCP "ef" for tagging site-to-site VOIP in order to prevent choppy voice qty. But we still got the issue intermittently. While further investigating, the ISP found there is no voice traffice with a tag 'ef' flowing so they want us to set up the 'ef' tagging in the phone system. But our phone system provider doesn't know how to. That makes us think maybe it should not be the phone system where to set up the tag. In our LAN we have an IAD which is ISP's property and is not accessible to us. Other than that are Cisco ASA, Switches, etc. None of them is configured with QoS.
So the question comes down to be: where to set up traffic tagging so the ISP's QoS can identify the "ef" tagging and prioritize it?