This might already be a known, given, or otherwise common knowledge, but I just stumbled upon it....
If you use CallWithUs like I do, you have a SIP trunk setup to make calls with them to certain destinations. And you would be right to assume the connection to CWU from your personal choice of SIP hardware/software is via tcp port 5060 to sip.callwithus.com. It is what my Pi reports in the SIP report, and it is what I found on my router on the NAT translation table.
But the actual call takes place via Level3. CallWithUs hands off your RTP to them, and it remains there for duration of the call.
For those of you prioritizing your VoIP calls ahead of everything else, please make sure you add Level3 address space to your QoS constructs. Their address space is 4.0.0.0-4.255.255.255.
Did this to my Cisco router and presto.... No more choppy calls when using CWU
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