The days of trying to make an important call from the bowels of a data center, only to find that your cell phone doesn't have a signal, may soon be coming to an end. New services and devices designed to extend the reach and reduce the cost of traditional cell phone service are beginning to reach the market, and are certain to change the industry's landscape.

Voice over Internet Protocol (VoIP) service providers Skype and Vonage have each recently partnered with hardware manufacturers to release cell-like phones that can use their services via Wi-Fi networks.

Not to be left behind, traditional cell carriers are launching dual-mode phones and services that run over the cellular networks, but switch to cheaper (for carriers), faster (for customers) Wi-Fi networks when one is available. T-Mobile launched a trial of this type of service, dubbed HotSpot@Home, in October 2006. Although the service is currently only available in Washington state, it's likely to be extended to other cities in 2007.

But before we get in too deep, let's get familiar with the technology and some terminology.

Wi-Fi phone services can be divided broadly into two categories: mobile phones that use a wireless network to connect to a VoIP service such as Skype or Vonage, and dual-mode phones that have the capability to run over both wireless networks and a cellular network. The goal of both of these approaches is the Holy Grail of fixed mobile convergence, which brings mobile and landline services together into a single device.

Netgear Wi-Fi Phone comparison chart

By the end of the second quarter of 2006, more than 9 million homes in the United States were using some sort of VoIP service, according to a December 2006 report from In-Stat. That number includes facilities-based providers, such as Vonage and the cable companies, as well as software-based providers such as Skype. Although the rate of growth for VoIP services is brisk, the next logical step is to offer alternatives to using the services only through a traditional handset or a headset tethered a PC. Enter Wi-Fi phones.

While dozens of telecom and networking companies have launched facilities-based VoIP services, the majority of these services are intended for use only within the confines of a home or business. They run over the customer's Internet connection and terminate in a traditional digital or analog handset. Several software-based VoIP services, such as Google Talk, Yahoo Messenger, Gizmo, and Skype, take this one step further by running on subscribers' PCs, allowing them to make free "calls" over their Internet connections to other users running the same software. A few of these services, most notably Skype, also offer an option to make and receive calls from landlines for an additional monthly or per-minute fee.

As for dual-mode phones that hop between wireless and cellular networks, the current leading approach uses Unlicensed Mobile Access (UMA) technology to handle the wireless side, with a seamless handoff to and from a traditional GSM/GPRS cellular network when a high-quality wireless signal isn't available. If the phone has an active voice or data session and an available wireless signal is detected, the phone will hop to the cheaper, faster wireless network. In theory, the handoffs back and forth between the two types of networks should be transparent to the subscriber.

UMA, which also is referred to as a Generic Access Network (GAN), uses the same slice of the broadcast spectrum -- 2.4 GHz -- used by many wireless networks, cordless phones, and Bluetooth devices. While there's also some question about whether or not UMA/GAN will remain the long-term solution for fixed mobile convergence, once dual-mode phones gain wider acceptance, it's certainly possible that changes to the network infrastructure will be made.