Many large communications vendors are still offering UM products based on the International Telecommunication Union 's H.323 protocol for enabling IP communication, because H.323 is far more mature than SIP and contains well-defined call-control features.

"Many products now include both H.323 and SIP. So legacy vendors may offer SIP enhancements to their current H.323 platform," says Elizabeth Herrell, an analyst at Forrester Research Inc. in Cambridge, Mass. Communication heavyweights now involved in SIP deployment include Cisco Systems Inc., Nortel Networks Ltd. and Avaya Inc., she says.

Maturing protocol

SIP is considered less complex than H.323. As the protocol matures, SIP will likely gain ground in the UM market -- something analysts are already starting to see, say Herrell and others. Currently, about 15 percent of major corporations have UM capabilities in place, but another 29 percent are now seriously considering the technology, according to Forrester.

The University of Miami's UM adoption was slower than expected. For years, the school had considered a major voice-over-IP investment, until IT officials decided in late 2005 to buy Communite, a UM/voice-mail replacement system from Indianapolis-based Interactive Intelligence Inc. Communite's reliance on SIP helped nudge Miami officials into action, and now about 14,000 users have access to UM capabilities, Seruya says.