It sounds like you're describing starting a group fictive video call and then nobody turning on their video. This is different than an audio FaceTime call. Try doing this with a single person and compare the audio, there is a noticeable difference though which is better is probably preference. There is then also a difference in how the OS treats the call.
So there two buttons, one that’s for group video call and one that’s for group audio calls. What you’re saying is that the latter is basically the same but without video? Damn
I dont think there is a single button where all the participants have their video off and only audio is on. The current way is to start a FaceTime video chat and then manually turn the video off.