Incoming calls audible during FaceTime on iPhone

While I was on the phone with someone on a FaceTime call, there was a car coming in on my side. They asked me what is that ringing is someone calling you? How are they able to hear my incoming call? That's not cool at all. Is it because my ringer was on my phone should be private on my end. Is it because someone is sending my calls and messages to their phone? Are they forwarding my information to their phone? How do I rectify this problem?

[Re-Titled by Moderator]

Original Title: FaceTime call