What could cause a glitch sound during a FaceTime call?

The scariest thing just happened, so I was on facetime with my boyfriend while i'm also just not touching my phone and doing something else on my computer when all of a sudden i heard this creepy glitch sound and i thought maybe it was his game or a video he's watching so i say out loud, what was that and i didn't hear a response so i said out loud, hello? and i heard a woman say "yes" and i hung up. I'm confused because when i looked at my phone it said that it was still me and my bfs call, we both checked our call logs to see if someone else called us but no one did and i asked him if he heard that and he didn't know what i was talking about.

Does anyone know what happened?

[Re-Titled by Moderator]