Why am I hearing a muffled stranger’s voice during a call?

I have an iPhone 17e, and my girlfriend has a Galaxy S26 Ultra. Regular cellular calls often fail or disconnect, so we've been using a calling app instead. While we were sitting quietly on a call, I suddenly heard a muffled voice that sounded like someone saying something about their leg hurting. My girlfriend didn't hear it and had no idea what I meant. About 10–20 minutes later, I heard what seemed to be the same voice again, followed by something like a throat clear. It happened a third time after she had started a video, and the sound had the same pitch and general quality each time. Could this be telephone crosstalk, audio bleeding from the video, or a glitch in the calling app? The internet connection is sometimes unstable, and I'm wondering whether there's a way to troubleshoot or prevent it.