Businesses willing to shell out £900 ($1,330) to upgrade their IP PBX now have a way to make free roaming calls using Skype running on the 3 network, PBK add-on maker VoSKY has said.

According to Silicon Valley startup, last week's news from the 3 network that it plans to allow subscribers to make zero cost Skype-to-Skype calls from its mobiles, offers businesses the opportunity to slash calling costs.

Buying one of its PBX-to-Skype gateways would allow roaming users to communicate with company offices for free, no matter where they were in the world. Office-to-office calls would also be free, as would calls from office workers inside an enterprise to employees roaming with mobiles.

VoSKY's innovation is that it can piggyback multiple user numbers within a company on to a single public Skype ID using mapping.

"Every VoSKY gateway has a Skype ID and it will show up on the network," said VoSKY's David Tang, after 3's announcement last week. "All you need now is a 3 phone and a VoSKY gateway and you have fixed-mobile convergence."

In the past, what had been missing was a way to route Internet-based VoIP calls such as Skype from mobile phones without also incurring at least some charge for the mobile-to-network cell connection. The announcement by 3 UK, removed a cost impediment to business use of such technology, said Tang.

Getting free calls requires users to run the client on a compatible phone, with a compatible 3 SIM, using a version of Skype downloaded from the 3 network. The required VoSKY hardware varies depending on the number of concurrent lines required and the type of PBX in question, but that PBX must be IP compatible.

The current entry-level VoSKY appliance is the Exchange 9040, costing £900, and capable of supporting four concurrent Skype lines. The company plans to replace this in the coming months with a more expensive model supporting more concurrent lines, the VIS8, costing £3,000, a company spokesperson said. However, the new model was totally plug-and-play, unlike the current one which required a PC for setup, the source said.