An audio interface like the Behringer UMC404HD can do this.
It requires to set a bus for mix-minus, let's say bus A
set this bus to UMC404HD Asio OUT 3

Now it requires a Y jack for the phone:
- mic input comes from UMC404HD OUT 3
- headphones out goes to one of the UMC404HD inputs, let's say input 1

Now in vMix add an audio input with UMC Asio input 1

Done!

PS : here's what I call a Y adaptor

via USB Asio driver (it's full duplex, works both ways)


The output #3 of the UMC becomes the input source for the phone. It's the mix-minus from vMix. That's the source audio for the remote caller to hear.

There are several ways to do this.
1) The simplest would be to get the guest to use vMix Call, since it automatically sends a mixminus feed back down the line.
2) Use Skype or any number or other free/paid audio services such as ipDTL, and then create a new bus on vMix to send back a mixminus.
3) The oldest way would be to use a Phone Hybrid device, which will take input from a wired phone line and output it by XLR cable etc. Second-hand units can be found cheaply now, it’s v old tech!

Yes, that's correct. They'll have to send a video signal, which of course you don't need to use. So preferably get them to use a (good) wifi rather than consume vast amounts of their data plan.