A carrier-grade voice over Wi-Fi (Vo-Fi) application will soon be available at the Apple App Store for the iPhone, as well as for Apple's Wi-Fi-enabled iPod Touch portable media player. The availability of the application should remove the risk of unpredictable and often sky-high cellular usage and roaming charges for international travelers.

The Vo-Fi application is coming as early as Feb. 1 from DeFi Mobile, a pioneer in using Wi-Fi to circumvent high cellular voice and data usage and roaming charges with a unified, worldwide, flat-fee Wi-Fi service. The company went commercial with its service last fall for Symbian-based handsets.

To provide service, it uses a sprawling Wi-Fi infrastructure comprising the majority of Wi-Fi hot spots around the world knit together by DeFi’s own carrier-grade backhaul network. The DeFi network peers with 200 of the world’s major landline WAN service providers with access to 2000 provider networks, according to CEO Jeff Rice.

iPhone or iTouch users who download the De-Fi app from the Apple App Store will get 101 free minutes of service. After the 101 minutes are up, they can continue to use the service for free with users of DeFi and other non-TDM voice services such as DeFi, Skype and GoogleTalk. Or they can click a button to upgrade to the DeFi Global Access service for $40 or $50 per month, says Rice.

For $40, they can make calls virtually anywhere globally where there is Wi-Fi coverage without having to log on in each hot spot. For $50, they can have up to three different phone numbers in different countries. For example, if a U.S.-based company has manufacturing facilities in China, a U.S. iPhone business user could provide his/her Chinese colleagues with a local Chinese landline phone number that rings his or her phone in San Francisco, bypassing the long-distance rates, Rice explains.

iPhone use for international business travelers has proven problematic for several reasons. The iPhone has been locked, not allowing users to swap out SIM cards that would allow them to get local cellular rates in the various countries to which they travel. Also, the device itself is “chatty,” sending background pings and updates transparent to the user, but using up chargeable minutes. And VoIP over the 3G cellular data channel is forbidden, both by Apple and in the user terms and conditions set by AT&T Mobility, Apple’s wireless carrier partner in the U.S.

Joanie Wexler is an independent networking technology writer/editor in Silicon Valley.