Yea, if they left you a voicemail it's best to call back ASAP. If they hadn't of called you first I would have said to wait a few days, but since they called it's best to call back. When that happened to me (they called me, left a VM, I called the next day), by the time I got back in contact (I had to leave a VM, then called a few days again when I didn't get a second call back) I found out that the position had already been filled! :(
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So when you're happy (Hurray!), or sad (Aw!),
Or frightened (Eeek!), or mad (Rats!)
An interjection starts a sentence right.