Are you leaving a message and they are not calling you back? Or are you hanging up when you get the voicemail?
A number of programs I've contacted (not all of them, but enough for me to share) have said they don't answer phones or respond to email until early the next day. Evidently that's the only way they can continue to stay productive during the day. Makes sense for programs with high applicant volume. So, I would leave a message.
But if you're leaving messages and they're not calling back, that's poor form on their part. No suggestions then.