The article explains why JS's message went to voice mail:
Also, I'll bet the police have the actual voicemail message, even though it was deleted. It's not impossible to get a deleted message, even though the Zahau family lawyer claims otherwise.
http://www.760kfmb.com/story/15485069/records-reveal-final-cell-phone-calls-to-rebecca-zahau
It might not be impossible but it has not been recovered.