Is that audio feed just your microphone, or is it a mix of the microphone plus other things? (And if so, what other things?)


What is "the audio" that you're feeding into a vMix audio input so the guest can hear it? What does it consist of, where does it come from, and how are you sending it to the guest? It's really not clear, but it might be related to your problems.


What sources are in the echo? Is the the caller's audio, or the contents of the feed that you're sending to them?


Again, What are you referring to as "the audio" here? And "back into the system" - from where? You can make any vMix audio input just a "monitor only" by only feeding it to the Headphone mix but NOT going to the Master or any of the busses/groups A-G, plus you should generally ensure that the audio device used by the Headphone mix is not also being shared/used by another vMix group (eg. Master), otherwise it gets difficult to work out what sources you're actually listening to. Then you can set up a headphone mix of whatever you want to listen to, independent of the Master. You can also do the same sort of thing by using any unused audio bus (A - G) as a monitor mix/group.