Industry analysts such as Herrell and Mahowald agree that the fact that SIP promises to one day knock down the price per UM seat is currently the most attractive aspect of the protocol, which was spun out of the Internet Engineering Task Force (IETF) as a signaling protocol for peer-to-peer multimedia applications.

"SIP may over time help vendors lower their cost structures," says Mahowald. "Since it has been ratified by the IETF, SIP has been generating a buzz. Many enterprises figure if they are buying UM solutions, they might as well be SIP-compliant. But SIP itself doesn't really make a tremendous amount of difference to users right now."

Sean McRae, vice president and chief technology officer at Prudential Northwest Properties in Portland, Ore., echoes that assessment when he describes his company's decision to purchase a SIP-based IP PBX system from 3Com Corp. in Marlboro, Mass. "SIP played a role in the process, but I don't really think about it. It's transparent," he says.

What companies do think about are the concrete returns on investment available through VOIP purchases -- ROI that can be enhanced easily by adding UM options, which are in turn made easier to deploy using SIP. For instance, officials at the National Institutes of Health Federal Credit Union in Rockville, Md., purchased Interactive Intelligence's Customer Interaction Center IP PBX, which is based on SIP technology, under the premise that the equipment would yield definitive returns plus "soft" UM gains.

"We expect to pay for the system in hard costs with the offset of additional maintenance, voice circuits and equipment," says Kirk Drake, vice president of IT. "However, the true ROI, we believe, is in the soft costs and the ability to keep our associates more directly connected to our members."