At this week’s Consumer Electronics Show in Las Vegas, at least three companies are introducing cordless phones that use a Wi-Fi connection to make calls over eBay’s Skype Internet phone service (a technology sometimes known as “VoIP,” for voice-over-Internet protocol).

Wireless-equipment firm Netgear and appliance makers Panasonic and Phillips all said they will bring out phones that add some measure of mobility to the experience of making a phone call over the Internet – an activity that has, for the most part, required users to wear a headset attached to a computer with an Internet connection. Panasonic said it would also introduce a Wi-Fi phone that works with Vonage, the leading Internet telephony company.

I’ve been using a similar phone with Vonage’s network, the UTStarcom F1000, which Vonage itself is selling for $79.99 (after a $50 rebate) plus $14.99 per month for the company’s Basic 500 Plan. This small, lightweight gizmo is a lot of fun if you live and work in buildings that have Wi-Fi networks. It’s also an easy way to try out the wonders of Vonage. But even though the F1000 looks like a little cell phone, it’s not really practical as a replacement for that device.

With its stubby little antenna, green and red “phone” buttons, and square one-inch monochrome dot-matrix screen, the F1000 resembles a 1999-era cell phone, except that it can’t transmit more than a few dozen meters. Fortunately, it doesn’t need to. Like a wireless laptop, this phone uses 802.11 Wi-Fi networking technology. When you place a call, the F1000 digitizes your voice into data packets that are sent to the nearest Wi-Fi base station.

The base station then sends the packets over the Internet to Vonage, which turns the packets back into the sound of your voice and completes the phone call. Because it doesn’t need as much transmission power as an ordinary cell phone, the F1000 has a battery life of two or three days, depending on how often you use it.

As a phone, the F1000 has some great features but an equal number are missing. You can use it to call any phone in the world, but it only works if you are within range of a friendly wireless network. The voice quality is a little compressed but not annoyingly so. The phone’s caller-ID feature displays the caller’s name or phone number, but not both. You can load the phone’s 200-number memory from the call log, but not from your PC. There’s a microjack to plug in a headset, but no built-in speakerphone. The phone charges with a standard mini-USB connector, but charging takes hours.

If you are interested in trying out Vonage, the F1000 is a great way to go – assuming you have a Wi-Fi network in your house. Just turn on the phone and register it with your wireless network. The phone will connect to Vonage, download the latest version of its firmware, and be ready to use within a few minutes.

Vonage has two residential plans: the Basic 500 gives you 500 minutes per month to call anywhere in the U.S., Canada, or Puerto Rico for $14.99 per month; additional minutes are 3.9 cents each. The company’s $24.99 per month Premium Plan gives unlimited calling within the same area. Both plans allow unlimited incoming calls and unlimited calls to any other Vonage phone. And Vonage has rock-bottom rates to elsewhere in the world – like 4 cents per minute to the United Kingdom, 10 cents per minute to South Africa, and 70 cents per minute to Afghanistan.

But what I really like about Vonage is the company’s extra services. You can call in for your voice-mail messages or get them delivered by e-mail. You can set your phone number to call-forward to any number in the world. You can even set up multiple phone numbers that ring simultaneously when someone dials your Vonage number. I moved to Vonage two years ago. When somebody dials my number, it rings my home, my cell phone, and my desk phone at Harvard. And when I pick up one phone the others automatically stop ringing.

Ironically, the F1000 falls down when it comes to working with the local wireless network. The phone has a wireless scanner and supports both 802.11 WEP and WPA encryption systems, so you can set it up with most home or corporate wireless networks. Unfortunately, the phone needs to be rebooted every time someone changes the configuration of the network, which makes managing the phone a real pain.

A related and even more serious problem is that the F1000 doesn’t have a built-in Web browser. This means that you can’t use the F1000 with any wireless network that requires you to type in a username, password, and credit-card number – which rules out T-Mobile’s HotSpot network, AT&T’s 802.11 wireless network, the Surf and Sip wireless network, and many more. Even many airports that offer free wireless service still require that users click a web button labeled “I agree” to accept the network’s terms and conditions. Alas, with the F1000 there is no way to agree.

So consumers shouldn’t count on a Wi-Fi phone like the Vonage F1000 to fill all the roles of a cell phone. But in some scenarios, Wi-Fi phones may be ideal – for example, as the main phone for the college student on a budget. Many university networks allow students and faculty to register their wireless devices using the device’s MAC (media access control) address, eliminating the need for a Web browser, username, or password. Then, for just $14.99 per month, the student gets a phone that can receive unlimited calls anywhere on campus and place calls to anywhere in the world at dirt cheap rates. Although it probably won’t work off campus, at least the voice messages can be delivered by e-mail.

Expect to see these next-generation cordless phones showing up in more pockets – and continuing to disrupt the old telephone companies’ business models.