[Boudica]

If It Is a threat, big mistake on his/her part to leave a voicemail.


First and foremost, make sure you backup the voicemail recording. If anything eventuates, you'll have grounds to take action. Whether It's a prank or a threat due to some sort of animosity or hostility, remains to be seen. I cannot be definitive with the Information at hand. Play It by ear and take It from there.

Well said, I'd stick to this advice.