Revenues from private branch exchanges that support the Internet protocol along with analog connections to the traditional telephone network increased 72 percent last year, a market research firm said Tuesday.

IP PBX revenues rose to $2.1 billion in 2004, while worldwide vendors of enterprise voice equipment shipped 46 million lines, a 5 percent increase over 2003, the Dell'Oro Group said. A large part of the latter increase was due to robust sales of IP PBX lines.

"The strong results for 2004 indicate that IP PBX technology has moved into the mainstream as enterprises now often select IP PBXs rather than traditional PBXs," Dell'Oro analyst Steve Raab said in a statement.

Enterprises are using the voice over IP (VOIP) features of IP PBX to support branch offices, rather than installing separate PBXs at remote locations, Raab said. In addition, IP PBXs are being used to enable sending and receiving of telephone calls from laptops.

In the fourth quarter of 2004, the top three vendors in PBX manufacturing were, in order from one to three, Avaya Inc., Siemens AG and Nortel Networks Corp.

In other Dell'Oro research, the analyst firm said revenues from IP telephony carrier equipment, including softswitches, media gateways, and hybrid media gateway softswitches, grew to $1.6 billion in 2004, a 26 percent increase over the prior year. A softswitch is a programmable network switch that can process the signaling for all types of packet protocols.

The top three vendors, in order from one to three, were Nortel, Sonus Networks Inc. and Tekelec.