As a point of information, the SIP protocol can negotiate an SDP session with any content, however both ends must agree. The PSTN, which is probably what you are limited to, has a bandwidth filter that rolls of steeply below 300Hz and above 3000Hz, it does not support stereo it was designed to best carry voice, not symphonies . If your phones are local and support a higher common codec then you will get that. It is all about the “highest common codec” that you prioritize and that can be negotiated between your endpoints, and that includes your VSP if involved.
I know this is an old post, but I am running into the same issue. Quality onhold is bad. I want highest quality possible. In the screenshot above 722 is not selected. If I select g722 and SLN48, I get dead air. Can you explain how to set the codec on the phone for G722? I have yealink T46S