AT&T has announced a business voice-over-IP service that delivers phone services over the company's IP backbone.

Although VoIP has been possible on the network for some time, the new offer makes it part of AT&T’s managed offering, and therefore a candidate for replacing customer’s traditional PBX telephone systems.

Users with international offices will be able to route all internal calls across the network, and make calls from anywhere to the US at local rates, since AT&T can terminate these calls on its US phone network. Similar deals for calls terminating in other countries are planned, but must wait for partnerships with local carriers.

The difference with this VoIP offering, compared with using an IP-based PBX in the company premises, is that there need be no PBX at all, as all calls are routed via a central facility.

“We used to replace the PBX in sites A and B with IP PBXs, or else add IP cards to existing PBXs,” said Martin Silman, director of global offer management at AT&T. “But why have PBXs everywhere? Why not have one on the network? – and we’ll host it.”

It sounds bizarre for a multinational company to manage calls between two adjacent rooms in Basingstoke, via a central “PBX”, hosted by a third party in London, (or New York, or wherever). But Silman claims this can be done reliably:

“A local call is set up by a short transaction to a remote call manager, which then plays no role until the call is torn down,” he said. “The local PBX is history, as long as you have resilience.” The resilience is provided by a Cisco router capability, called SRST, he said.

The scheme sounds far more reasonable, when Silman points out that calls are actually between people rather than phones. The central call manager would be able to route the call between workers at neighbouring desks, even when one of them is actually on the road, at home, or at an office the other side of the world.

“You can behave as if you are in your office, wherever you are,” said Silman. “You can use a softphone on your PC, when you are at home on broadband, and still be on the corporate numbering system.”

“We have offered voice over data for three or four years. That much is business as usual,” said Silman. “Now, we intend to cover all the features such as IP conferencing, and unified messaging.”

Unlike previous VoIP offerings, it comes with a global service level agreement. There's also a customer portal where users can manage and monitor what quality they are actually getting, allocating bandwidth between voice and data services, and even drilling down to individual calls, he added.

AT&T itself is already using the service, having moved 1,000 of its 3,000 UK employees to it in one year.

Europe is expected to take it up quicker than the US, as it is based on the multi-protocol label switching (MPLS) backbone, use of which is about 18 months more advanced here, said Silman.

“In the US, there is a massive base of frame relay customers, whose propensity to move is slower than I’d like or expect. In Europe, offering frame relay is almost suicidal, as customers expect MPLS. MPLS has become the new frame relay, here.”