The new offering, called Skype for SIP, uses Session Initiation Protocol (SIP) to exchange signaling between PBXes (private branch exchanges) and the free or inexpensive voice service. The latest PBXes support SIP, a signaling protocol designed to span IP-based phones, software clients and other components of unified communications systems.

Business users will be able to make and take Skype VOIP (voice over Internet Protocol) calls using all the features of their internal switches, according to the company, a division of eBay. Enterprises are widely adopting IP-based phone switches as they replace aging circuit-switched units.

Some vendors already offer equipment or services to link Skype's network to other types of telephone system. Skype's latest move, however, could enable its customers to do without such third-party products.

Skype is letting businesses that have SIP-based IP PBXes to register to test a beta version of the service, but warned that the number of beta testers will be limited. The full version of Skype for SIP will launch later this year. Support for beta testers will only be in English, Skype said.

Businesses can register their SIP address with Skype and pick what calls they wanted routed over Skype, said Ian Robin, heads of sales and marketing of Skype for Business. Those testers will only have to pay for the calls they make during the beta test period, Robin said. Skype hasn't determined how much it will charge after the beta test ends.

Skype will stick to the same international calling rates that it offers for its consumer service. Calls to the U.S., for example, are US$0.021 or $0.024 including VAT (value-added tax), which is levied in Europe.

Many enterprises that have bought IP PBXes aren't actually taking advantage of VoIP, according to In-Stat analyst David Lemelin. They run hybrid internal voice systems and continue to use expensive leased lines for calls into and out of the enterprise, he said.

About 438,000 IP-enabled PBXes were shipped in 2008, wrote Peter Parkes on Skype's blog, citing figures from analyst IDC.

Companies will be able to manage Skype calls with existing hardware and applications that perform functions such as call routing, conferencing, voice mail and call recording, Parkes wrote.

Skype's business package will also allow companies to put a "click-to-call" button on their Web site. When clicked, a call will be routed via Skype through to the company's PBX system.