If you're a business traveler making frequent trips overseas, you know tracking your roaming phone calls is like trying to herd cats; it can get out of hand fast. DeFi Mobile wants to ease that pain by allowing unlimited, international calls -- sent and received -- over Wi-Fi-equipped phones. That's not all. The cost is just $40 a month, flat.

For it to work, you need a phone with the DeFi software installed. Currently it only runs on select Nokia handsets like the Nokia 95, N82, E90, those with integrated Wi-Fi, and the Symbian S60 operating system. If you don't have one -- and if you live in the U.S., chances are slim -- DeFi suggests buying one unlocked at partner Communication Xperts (lowest price for a DeFi-friendly handset: $325 for Nokia e51).

DeFi has relationships with Wi-Fi network providers like FON, T-Mobile, AT&T, and many more, especially with aggregators of such networks in Europe (the company is in early talks with U.S. aggregators like iPass and Boingo). When the phone comes within range of a Wi-Fi access point, it knows -- say at your home, office, or a public hotspot that has a DeFi presence -- by auto connecting.

You receive one phone number for your $40. Spend an extra $10 a month to get three phone numbers, each registered for different countries. That way people calling you may make a local call to your phone -- even if they're in London or Madrid, but you're in San Francisco. Any call to any of your three numbers forwards to that same phone. If you are not within range of a Wi-Fi node you can't take a call, but there's voice mail accessible via a Web interface.

With no Wi-Fi--if you've got service with a GSM provider like AT&T or T-Mobile--you can still use the Nokia phones (subject to the carrier fees). There isn't any seamless hand-off like with T-Mobile @Home. In fact, CEO Jeff Rice says biz-traveling beta users apparently didn't want that feature, preferring to know precisely when they switched from Wi-Fi to GSM.

Making outgoing calls on Wi-Fi via DeFi is meant to be indistinguishable from dialing out on the cellular network. It could keep DeFi from working on certain platforms, specifically the VoIP-unfriendly iPhone (but it's too soon to say yet). Work is definitely underway to bring the DeFi software/service to Android and Windows Mobile phones in the future.