A new study says sales of IP PBXs have outstripped those of traditional TDM PBXs and that trend is likely to continue.

In fact, by 2009, IP PBXs will represent 91 percent of all PBX sales worldwide, according to "IP PBXs: Emerging Into Dominance" says In-Stat.

The surge by IP PBXs is even more remarkable because the total sales of PBXs will grow at 6.6 percent per year over the same period, the study says.

The growth in PBX sales is measured in numbers of lines shipped worldwide, which the report projects will grow from 9.5 million this year to 28.1 million in 2009. In the VoIP world, a line is defined as an end-user license.

While sales of IP PBXs will overwhelm TDM PBXs in terms of sales, TDM equipment in use will still outstrip VoIP PBXs for quite some time, says the author of the report, Norm Bogen. That is because traditional PBXs and the phones used with them can stay in use while businesses that own them migrate to VoIP.

VoIP PBXs can be fitted with VoIP gateways that can connect islands of TDM to larger VoIP networks, Bogen says.

Integrating voice with data via IP PBXs gives corporate customers new ways of doing business, which is helping drive demand for the devices, he says.

Via these platforms, businesses are adding presence capabilities to their communications networks, the study says, which gives workers more flexibility about when and how they can be reached. It will also support the addition of video to corporate communications networks, it says, boosting the effectiveness of collaboration tools.

Bogen says the desire for wireless phones within businesses will also drive the adoption of VoIP. With dual VoIP/cellular phones, users can make calls over a wireless LAN and through a corporate IP PBX for less than a cell call costs.

When they leave the building and make a call the phone uses the public cell networks. Some vendors make phones that can roam between a Wi-Fi connection and a cellular connection on the fly, making a handoff that users cannot detect.