Companies with voice over IP technology keep hold of their traditional voice systems - and invest in multiple technologies, according to a survey of US users.

Around a fifth of US business have VoIP, and adoption will continue to grow at a robust rate, according to data from vendors and carriers, said David Lemelin, an analyst at research firm In-Stat. Two-thirds of US businesses will have VoIP service by 2011, he said.

Despite adopting VoIP, users are holding onto traditional voice communications technology for part of their operations. Traditional time division multiplexing, or TDM, still accounts for 44 percent of the voice lines in the 20 percent of US business that have adopted VoIP, said Lemelin.

Businesses are also relying on multiple flavours of VoIP, said Lemelin. As well as buying IP-PBX switches from vendors such as Cisco, they are upgrading their TDM switches to allow IP telephony on the TDM lines, he said.

To add to the variety, businesses are also buying VoIP services from traditional carriers, Inc., and allowing individual workers to set up client-based VoIP systems that do not rely on a central switch, such as SkypeIn and SkypeOut, Lemelin said. In such systems, users load client software and connect to the public telephone network.

Yet another VoIP flavour that is finding favour is voice-enabled instant messaging, in which a user adds voice to a traditional text-based IM service, Lemelin said. "Increasingly, we're finding a lot of businesses are using voice-enabled IM," he said. "It's just one more option." About 14 percent of US business have at least some workers using voice-enabled IM for business purposes, he said.

"As people become more savvy with VoIP options and as the Generation Y worker enters the workforce, they will be very comfortable with multiple VoIP options," Lemelin said.

The implications for IT managers in businesses are fairly clear, Lemelin said. "IT managers will have to decide whether or not they will allow all these flavours, and I think there's an opportunity for the client-based providers to begin offering packages or bundled VoIP solutions that would be more business-grade," he said. By "business-grade," Lemelin said he means a more secure system than some consumer-focused products.

Overall, about 36 percent of the business that adopted VoIP are using a variety of VoIP systems, he said.