Interfacing a physical phone is possible. There are devices to make this practical. JK Audio is one brand to consider.

I would not use a hardware solution. I used a soft phone. For years I hosted a live podcast/stream* that had call-in from all around the world. We used a soft phone to dial into a conference bridge. Participants could thus participate in their own voice.

This is a lot like vMix Call, voice-only, where the remote party can be on a real telephone, mobile phone or computer. The conference service I used allows this.

Using a soft phone requires virtual audio cables to route the audio to/from vmix. I also used VoiceMeeter, since back then the internal audio routing in vMix was limited. It's much better now.

Using a conference service gave us a lot of tools. We could have multiple guests connected. Breakout rooms allowed us to have private chats with them, like a green room.

I wrote this all up in a few blog posts.

*I'd link to the site for that project, but it was lost to a major server upgrade when my former partner failed to make backups. It did run for about 13 years, and 700+ episodes. The YouTube library remains. It has all the shows that involved a Google Hangout.