which audio bus are you sending back to your caller?

on which audio bus are you receiving your caller?

where does the audio signal the caller is supposed to hear come from?

I would set up the following
Audio Busses
A - Recording
B - Return Bus for the callers
C - Listen Bus for your Headset (optional)

Call 1
AudioSource - B
Playout Bus - A, B, C(optional)


vMix doesn´t send back incoming call audio to its own bus

Headset
Playout Bus A,B
Listen Bus A (so you will hear yourself due to recording monitoring)
Listen Bus C (you won´t hear yourself, lacking complete recording monitoring)

If you use (M,A,B) you don´t have the possibility to only hear the callers,
cause when your headset is defined as "Headset" in audio output settings you will always hear M
over your headset

But there's no rule that says you have to use the "Headphones" option for headphones.

Before they added the additional buses, I used it for a virtual cable.

After they added the additional buses, I set it to "None" and put my headphones on Bus C.

You can set your headphones to which ever output bus you want--you can even set them to multiple buses so all the audio flows to your headphones if you want.