Early Wi-Fi-to-cellular systems focused more on large businesses, but these new phones use Unlicensed Mobile Access (UMA) technology to hand off voice calls between the GSM cellular network and a non-carrier-specific local area network (including both Bluetooth and Wi-Fi). These new UMA-equipped handsets won't cost much more, if any more, than standard models--and they can save you a bundle on airtime costs and service headaches.

The Wide World of Wireless

T-Mobile is the first nationwide provider to offer UMA, under its HotSpot @Home program. Cincinnati Bell is offering a similar service as a local provider. Both services let you add the Wi-Fi feature for US$10 a month over standard voice plans. T-Mobile even goes so far as to supply an optimized D-Link or Linksys router to simplify taking calls over Wi-Fi.

With either service, you can make Wi-Fi calls from your home network, from the provider's own hotspot networks, or from third-party Wi-Fi networks (you'll have to manually configure the handset to tap into these). When your call hops to the wireless network, you won't be charged airtime use as you would if you were using the cellular network.

Wi-Fi calling can be a major boon for international travelers: If you're overseas and using a Wi-Fi network, calls to U.S. numbers will be treated as normal (you'll get hit with roaming fees only if you call an international number). It can also benefit businesses, which already have wireless networks in place.

AT&T, the nation's other GSM network, hasn't formally announced UMA service, but that offering may be forthcoming. Later this summer the company will be carrying RIM's BlackBerry 8820, which is particularly well suited for data use as well as voice over a Wi-Fi network. For one thing, the 8820 is a typical BlackBerry, with a dedicated keyboard and a screen large enough for comfortable e-mail and Web surfing. (The Nokia and Samsung phones lack a keyboard and a roomy screen.) For another, the device includes 802.11a as well as 802.11b and g; 802.11a tends to have less interference than b and g do, and it can provide better compatibility with business networks.