Microsoft is expanding its work with enterprise telephony vendors to make its Office Communication Server (OCS) 2007 work more closely with office phone systems.

On Tuesday, at the launch of OCS, the company plans to unveil a formal program to certify interoperability between IP phone systems and OCS. As part of that, Microsoft will discuss a specification to let enterprises migrate one building at a time to its software-based unified communications system and still have calls go across the organization as if on the same PBX. Two models of Cisco Systems' popular ISR (Integrated Services Router) branch-office platform will be among the products certified for this type of interoperability, according to Zig Serafin, general manager of Microsoft's Unified Communications group.

Microsoft's initiative, called the OCS 2007 Open Interoperability Program, will formalize work that has already been going on with some third parties. As that work has expanded, it's reached a point where it needs to be more organized, Serafin said. The idea is to let customers know what will work with OCS, and Microsoft will provide a table on its Web site where potential customers can check the certifications of third-party products.

Although promoted as an effort to coexist with the IP phone systems now established or taking root in enterprises, the program also will make it easier for customers to migrate away from dedicated communications systems and phones themselves, the company acknowledges. Voice call control is new to Microsoft's unified communications system with OCS 2007, but the software giant envisions a day when separate platforms such as Cisco's CallManager won't be needed, industry analysts say.

Cisco, Avaya, and other vendors have already moved the voice call-control functions of traditional circuit-switched PBXes into server software, but they sell that software along with IP handsets and other gear. Microsoft intends OCS, together with Office Communicator 2007 client software or special OCS phones made by Polycom and LG Electronics, to ultimately replace those dedicated systems.

There are three methods of interoperability that will be certified under the program.

-- SIP CSTA (Computer Supported Telephony Applications) is based on a standard by the European Communications Management Association (ECMA). It lets users control calls through the Office Communicator client on the PC, though in most cases still using the handset and PBX.

-- OCS Coexistence lets the user pick up a call on either the existing handset or a client that uses OCS, namely Office Communicator or a special OCS phone.

-- Direct SIP interoperability allows for some parts of an enterprise to use traditional or IP PBXes and others to use OCS, with transparent connections between them using gateways, according to Microsoft. SIP is the emerging standard protocol for exchanging information on voice, videoconferencing, and other communications sessions.

Microsoft has already certified gateway products from five vendors for Direct SIP interoperability, Serafin said. Among them are Cisco's Integrated Services Router 2851 and 3845. In fact, all ISRs with voice capability can interoperate with OCS, according to Mike Wood, director of product marketing in Cisco's access routing group. Gateways from Dialogic also have already been certified.

As a newcomer to telephony, Microsoft will take time to displace many stand-alone telephony systems, so interoperability will be critical, analysts said.

Most enterprises that adopt OCS still have phones connected to PBXes and will dial through the PBX, said Brent Kelly, a senior analyst at Wainhouse Research. To start, most OCS users will keep their PBXes in place and take advantage of CSTA to gain the click-to-call benefits of OCS, he said.