Applied for a popular position, missed their call, called back and left a message. When should I call again?

Hey all so yesterday I applied for a popular job that I really want/need. Today I got a call back from them but I was taking a test so I wasn’t able to call back for another 20 minutes. When I called back the HR department was busy. I left a message but again, it’s a popular job at a very busy and nice hotel and I don’t want to lose this opportunity. I’m not sure if/when they might call me back. What’s an appropriate amount of time to wait for me to call them again?

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