The true value of convergence won't be realized until self-contained corporate VoIP networks are linked in to the larger IP world through carriers, network professionals said at last week's Fall VON 2005 Conference & Expo .

Some of these professionals say they are interested in tying their converged networks directly in to carrier facilities through IP trunks that use Session Initiation Protocol for transport. That would give businesses more flexibility in provisioning voice/data lines to facilities, while opening the possibilities of more-advanced applications, said those at the show, which attracted more than 300 exhibitors, 7,600 attendees and keynote speakers from companies such as BellSouth, Skype and Vonage.

But such deployments are in their infancy, as large IP trunks in carrier networks are hard to find.

"SIP trunking holds a tremendous value for tying a bunch of small sites together transparently," says Arnold Solomon, IT architect for the Southern Company, which operates utilities Mississippi Power, Gulf Power and Georgia Power, and provides wireless network services in the Southeast.

Most companies, whether they run traditional or IP PBXs, must plug these boxes in to digital public switched telephone network (PSTN) circuits from phone companies to make calls to the outside world. Companies might be able to connect branch offices and even home workers in to a unified VoIP network, but the VoIP stops at the voice carrier, in most cases, as it's converted to digital TDM voice.

The alternative is plugging in to an IP voice circuit instead of a less-flexible digital voice trunk. Solomon says if IP-based trunks connected the Southern Company's hundreds of offices and facilities, the firm could more efficiently manage how it buys and uses voice services, rather than having to purchase a local T-1 digital PSTN trunk to each site.

But Solomon adds that his own company still has far to go in terms of converging its internal networks before linking to a carrier via IP. With a mostly TDM-based infrastructure of Siemens PBXs, Solomon says the Southern Company is just starting to move toward IP handsets and PBXs.

Another user further along in its convergence push also seeks to take the next step with IP trunking.

"Local trunks are something that we really want to get away from," says Todd Goodyear, vice president and manager of voice product development at Merrill Lynch in New York. "Having [voice] T-1s locally at every branch [is inefficient]. We want to move from those to IP trunks."

Merrill Lynch is deploying IP phones to more than 600 branch offices this year, tying the sites back to centralized Cisco IP PBX clusters at distributed data centers. But remote sites will still require local trunks to make outside calls in some cases. Goodyear says IP trunking would let Merrill Lynch allocate PSTN phone trunks virtually through the company's centralized data centers. This would make management and provisioning easier, because data and on-net calls are already managed centrally.

"This is something we expected to start to have legs last year, but we don't see it taking off, whether it's issues around regulation or just demand in the marketplace," he says.

One expert says while carriers might be dragging their heels about offering IP voice trunks in their networks, such a move will ultimately benefit both customers and service providers.

"In the core of their voice networks, carriers are pretty much IP," says Annabel Dodd, an independent network consultant and author of the forthcoming The Essential Guide to Telecommunications, Fourth Edition. "Carriers are starting to move IP to the edge. It's just a matter of time before they do offer that on a large scale, because they don't want to continue to support two or three different types of networks."

For their part, vendors announced services last week at VON that might begin to address some of the needs for large-scale IP trunking and VoIP services. MCI and Sprint, for example, both announced services designed to tie together IP services and IP PBX gear.

Qwest and Microsoft also could help make converged applications more easily deployable. Through a new partnership, Microsoft said last week it is contributing its Solution for Enhanced VoIP services to a suite comprising hosted versions of Microsoft server products, including Exchange Server 2003, Office Live Communications Server 2005 and Windows SharePoint Services, with Sylantro Systems' Application Feature Server. Microsoft and Qwest are working to deploy this suite with Qwest's OneFlex VoIP network services.

Senior Editor Tim Greene and the IDG News Service contributed to this story.