Why does Google Voice sometimes give a busy signal after ringing?

I sometimes use Google Voice to call someone internationally, and sometimes it will ring and then give a busy signal. If I call back right away without Google Voice, it will ring and the person will pick up. Ringing followed by a busy signal seems odd and seems like it might be a Google Voice issue. Has anyone experienced this, and is it a real Google Voice issue?|||Google Voice still has some minor issues like that, but those issues should go away over time.