Beyond balancing pure VoIP and TDM hybrids, IT has another strategic decision to make, between vendors that consider VoIP an application, independent of the network data infrastructure, and those that sell the data network infrastructure as part of the overall package. In one camp, Alcatel, Cisco Systems and Nortel push their data-networking products as part of the VoIP system. Avaya, Mitel Networks and Siemens emphasize their products' ability to run on heterogeneous network infrastructures and may even partner with network-equipment vendors like Enterasys Networks, Extreme Networks, Foundry Networks and Hewlett-Packard.
There are benefits to sticking with one vendor, but before buying into a proprietary system, weigh the trade-offs. Your legacy PBX doesn't require a single data-infrastructure vendor, and neither do the business applications that run on your network. Why should VoIP be any different?
Beyond proprietary feature sets, there are other ways to get locked into one vendor. For example, many enterprise phone systems require the phones and the switch to come from the same source. This is nothing new--PBX vendors have always insisted that you buy high-end digital sets from them. For simple, single-line phones, though, there was always the option of getting an inexpensive analog device that works on any vendor's PBX.
Despite the fact that Ethernet and IP have made it easy to standardize the lower layers of IP phone connectivity, the signaling is often still proprietary. At one time, a number of standards were contending to solve this problem, but the IETF's SIP (Session Initiation Protocol) has emerged as the winner. Many vendors are balking at adopting SIP, claiming it's still immature. Although some advanced features must still be nailed down, standards-based SIP phones are more functional than standard analog phones. For example, many have LCD screens with programmable soft keys, something usually found only on high-end digital sets. This hasn't gone unnoticed by carriers and ISPs getting into the VoIP services market--you'll be hard-pressed to find one that isn't using SIP technology to roll out services.