Join by Phone Call -- Where to begin?

Hey there. Google Meet and other platforms feature a “join by phone” or “dial-in” option where you can call a number, enter a code, and be in the meeting with voice only. I would like to explore the possibilities of doing this for a mediasoup-based meeting.

I understand this isn’t something mediasoup supports itself. I am just looking for pointers on where to begin with this. What technology does Google Meet use to accomplish this? What sorts of terms and technologies should I be researching? I’ve been researching SIP and PSTN but I can’t see how it all comes together.

If anyone can provide some direction I would greatly appreciate it.