vMix Call is awesome and what I am about to propose may already work in some ways but I think a dedicated solution for comms would be awesome and your almost all the way there anyway.

Again, I realize there are many ways to skin a cat, but I haven't seen one that solves the problems below. If there are, I am open to suggestions or being pointed in the right direction.

Problem to be solved:
1. Comms are tricky, expensive, flaky, and just not simple to setup for the lower budget type of guy.
2. vMix call is one way to solve for some of the above, but it is overkill for a simple comms solution as video is not needed and may take too many resources for just audio
3. vMix call does work if you are ok with #2 above but if using a phone, in my experience on my android the screen needs to stay on for you to be able to keep using the microphone on the phone. In my experience (it could be a phone setting I don't know how to change) if I turn my screen off and put the phone in my pocket while wearing headphones (TRRS) the microphone only works for about a minute and then I lose the microphone ability. I can still hear the audio bus I selected but my mic doesn't work.
4. When using a phone, if someone calls your phone it is really annoying!

Solutions:
1. Copy and paste the vMix call program but delete the video portion of program and then somehow keep the mic always on even when the screen is turned off. This will save battery power for a long show and decrease bandwidth being used between the two. Disallow phone calls while using it, or at least an option to turn it off.
2. Make an actual android and iphone app that we could use for this specific purpose. I wouldn't mind paying for the app separately and it could be another source of income for vMix.
3. Variation of #1 above would be to make some changes in vmix call to turn off all video and only use audio, vmix would basically treat it as an audio input AND then make the website address only reflect the comms options to reduce possible bandwidth.

+1 for Unity Intercom - we've integrated that app across our production (both commercial and live broadcast) with excellent results