I am pretty sure I recognize the hold music. I believe it is from uberconference.com. You likely found one of their teleconference lines. Each paid UC user gets

I am pretty sure I recognize the hold music. I believe it is from uberconference.com. You likely found one of their teleconference lines. Each paid UC user gets their own local number. When a conference is active, dialing it will connect you immediately to the conference. (Of course, the host can chose to require a PIN, but I never had any unknown drop ins.) It is SO much better than Zoom’s system.