Disembodied voice coming through on the call!

I was on a call with my therapist just now and I could hear a male voice talking, although there was no one else on the zoom call with us. It sounded as if he was talking to someone else (like he was on another call) and couldn't hear us. This lasted for about 15 minutes, while I closed all other taps and apps on my computer, thinking it could be a video playing or another programme running in the background. The voice continued intermittently, and eventually we ended the call and continued on another platform. Very freaky! How did this happen, and are there any settings I should be using to prevent this?