Smart Voicemail is an Advanced Voicemail with a Virtual Assistant for smartphones and telephones. The major differentiator is that it has 'status updates & availability' to quickly let the callers know why you missed their call in contrast to a static Voicemail greeting that was recorded months ago. These status updates can be from Organizational UC / presence server or Facebook / Twitter or from your personal / work calendar.
It's smart enough to suggest and guide you throughout the day of packed schedules.
It's also viewed as an Out-Of-Office / Vacation responder for Smartphones.
Not just that, it also understands the context of every communication and help users organize their communications better.