When should I call back again?!?!?

I am a recent RN graduate and finally got my first call back to schedule an interview. Heres my story.... The nurse manager called yesterday morning from a hospital where I put an application in at while I was in the shower and left a message. In the message she said to call her back and she wanted to set up an interview. I called back 30 minutes later and left a message. She didn't return my call. When is the proper time to call back again? What should I say?

Thanks so much for all your help!