Sue,
Sometimes when the line is busy, it automatically goes to the anserwing machine. At least that is what happens at my company.
I am on the phone a lot and my machine kicks in when I'm on the line. It doesn't give a busy signal.

If I were you, I would try to call one more time before the end of the work day (5 pm.) and just say:

Hello, my name is ________, I am returning mrs./ mr. blabla's call in reference to my resume/job application. My phone number is _________________. I will try to reach you again tomorrow. Thank you.