When I use Ring Out the person I'm calling doesn't answer so i get voicemail. at the same time they pick up and I hear the voicemail in the background of my call with them.

When I use Ring Out and the person I'm calling doesn't answer immediately the call transfer to their voicemail.  At the same time they pick up and I hear the voicemail in the background of my call with them.  Why is voicemail and the live call for person called overlap?