Once your housing information was processed, you were assigned on on-campus phone line. There are directions here for how to access this voicemail account:
http://www.butler.edu/ir/?pg=3571
The voicemail you got is probably the all-campus alert system telling you the power was out a few days ago due to a blown transformer. So far, I have never checked my Butler voicemail because no one knows the number: I assume that it it's anything remotely important, my cell phone will be called so I really wouldn't worry abou the emails telling you about your voicemail.
Jess