Ora aqui está um documento bastante interessante que foi recentemente disponibilizado pela Microsoft: Voicemail: A story of unrealized potential.
This whitepaper details the value that Exchange 2010's Unified Messaging capabilities offer to empower your users, reduce cost and manage risk by replacing your voicemail system.
Voicemail is a relatively mature technology. It is so thoroughly established in users’ daily workflows that it has become practically invisible to the organization except as an ongoing expense. Because of their origins in telephony, voicemail systems tend to exist in their own silos, managed separately from other IT systems in general, and business messaging in particular. Many fail to provide the features and ease of use that users have come to expect from other types of messaging.
Yet voicemail is a critical communication tool for most workers. Given the right strategy, it can go from a basic service to a productivity-enhancing tool. One way to do this is to treat voicemail as just one more messaging modality. It is then possible to manage it using the same tools administrators and users know from email. A universal inbox is also the foundation for delivering advanced voicemail features across devices and platforms. Microsoft calls this capability Unified Messaging. Microsoft® Exchange Server is the key technology that enables it. The newest release, Microsoft Exchange Server 2010, delivers voicemail functionality engineered from the ground up to improve productivity while helping you reduce the cost and complexity of your voicemail system.