I do this
literally every week, and have done for about seven years.
First - you said "phone calls" so I presume that means you want your guests to be able to dial a normal phone number and reach you, with no other technical complexity. Ubiquitous, easy access.
My situation is a little different, since I allow anyone to join the show by telephone. Not just a single, planned guest.
I use a SIP soft phone installed on my vMix host. It connects to
a conference service (http://www.zipdx.info.) The audio to/from the SIP soft phone is dealt with using the same mix-minus technique that people have long used to receive Skype into vMix.
My "guests" dial into the conference service and enter a conference code. They are then connected to my show.
To keep noise to a minimum, I set the conference service to keep everyone muted at first. So they are listen-only. They can unmute themselves by pressing *6 on the telephone keypad if they want to speak.
The conference service has access numbers in about 60 countries, which makes it easier to have guests connect from far-off locations. It can also call out to them, which is sometimes cheaper and more convenient.
The conference dashboard has a great set of controls for managing the participants connected by telephone. I think of it as the stage manager & green room.
If you only need a way to connect to a single guest by telephone, you may not need the conference service. Any SIP-based phone service (there are dozens, like Vonage, OnSIP, etc.) That would give you a phone number that you can bring into your vMix system.
As a practical matter, given typical billing models, it can be both cheaper and more scalable to use ZipDX.
Aside #1 -
back in 2008 I interviewed the founder and CEO of ZipDX, which is how we met. He eventually sponsored our show, providing the ZipDX platform for our use which continues to this day.
Aside #2 - In 2013 ZipDX ended up hiring me.
Aside #3 - ZipDX supports wideband telephony (aka HDVoice,) which delivers far better audio quality. However, this post is already long enough.