As companies made / make the move from traditional POTS or analog phone services to VoIP (voice over IP) communications, they introduce new risks that may lead to sensitive communications being compromised. There are plenty of books available detailing various VoIP threats and potential attacks. Unified communications introduces the concept of Presence and expands communications beyond the phone, but is primarily built on or around voice communications- generally VoIP.
Securing communications is critical for most companies and is mandated by various regulations and guidelines. UC complicates things further by blurring the line between voice, email, instant messaging, network data, voicemail, etc.. A recent article from Network World explores the cracks that UC introduces into corporate network security as well as examining some of the myths or hype around VoIP security issues.