Frontier airlines absolutely does this. I think it's only during hours when there are not agents working, because I was able to reach an agent when I called ag

Frontier airlines absolutely does this. I think it's only during hours when there are not agents working, because I was able to reach an agent when I called again during more normal business hours. But it did feel really weird hearing the automated system say "that is not a valid option. goodbye" and hang up on me. No indication that I could call back during certain hours to reach a human.