"Bad guys need to constantly find new ways to lure their victims. If billing notifications were very common for a while, not all people in a company are working with such kind of documents. Which types of notification do they have in common? All of them have a phone number and with modern communication channels... everybody can receive a mail with a voice mail notification. Even residential systems can deliver voice message notifications."
Bad guys are turning to disguising their ransomware in email notifications that contain voicemail records. Take a look at this blog post from KnowBe4 and learn more and educate yourself, and your staff, to be on the lookout for this new "trick" that is out in the wild.