The success of voice over Wi-Fi (now officially called "Vo-Fi") depends on a number of factors, both technical and business-oriented. Somewhere very near the top of the list are the need for dual-mode cellular/Wi-Fi phones (with decent battery life) and integrated services from carriers.

We need one device
For example, who's really willing to juggle multiple phones - a cellular phone and a Wi-Fi phone, for example - for use depending on whether you are at home, at the office, or on the roacd? Business users getting comfy with mobility will quickly begin to pine for a single device that they can carry around for all their voice, messaging, calendaring and contact applications.

Without integration, the number of gadgets to keep track of could quickly pile up, depending on the role you are playing and your location. Each would likely contain overlapping contact and calendaring information but in different formats, driving the user to distraction.

The good news is that there is evidence that dual-mode Wi-Fi/cellular phones are en route.

... and here they come
Motorola says its CN620, its long-in-development dual-mode 802.11a VoIP/GSM handset, is imminent and should not be held up by development partner Proxim's recent Chapter 11 filing (and purchase by Terabeam). The CN620 will be distributed by its third development partner, Avaya.

And Nokia has said that it will license Cisco technology to integrate dual-mode Nokia Series 60 handsets with Cisco's CallManager IP PBX over Wi-Fi. Nokia, in a press release, said it intends to begin "to extend the highly valuable services of the PBX to Nokia mobile devices as well as enable use of enterprises' private infrastructure for part of mobile voice traffic."

This is a very interesting and telling statement. Typically, there have been at least two phone networks: the one you use at work, with all its custom, rich features and the public network, which works differently (and, possibly a third: softphone client-plus-Internet). At least two voicemail systems to check, two sets of features to learn and use. Blending Wi-Fi and cellular connections with enterprise PBX calling features in a VoIP handset is approaching telephony nirvana.

But how converged are they?
Some dual-mode devices with both Wi-Fi and cellular connections will simply let you connect to one type of network or the other. Others will automatically switch you to the fastest network within range, maintaining your session seamlessly as you roam across a network boundary.

This is where the mobile operators and service aggregators have a big opportunity (or should that be "responsibility," particularly in the case of voice?) to make things work. For the dual-mode devices that support inter-network roaming while maintaining sessions, will carriers be willing to seamlessly hand off users from, say, a cellular network to a Wi-Fi hot spot if that hot spot isn't their own (thereby handing off the associated revenue-generating minutes)?

Dual-mode means dual bills
By way of example, the HP iPAQ PocketPC h6320/6325 can be used on GSM/GPRS networks. Though it is not a Vo-Fi phone, the device also supports Wi-Fi connections (and with the addition of a softphone like CiceroPhone (reviewed here).

The h6320/6325 will indeed automatically connect you to the fastest available network - be it cellular or a Wi-Fi hot spot - regardless of whose Wi-Fi network it is. But here's the catch: in the US, the device isn't available from Cingular, a Cingular spokesman confirmed. You buy the device through HP sales channels, and Cingular will be happy to collect on any airtime it happens to accrue from that sale. But will it sell you a service package with the HP device? Not today, anyway. Meantime, you have to gather up multiple services (and monthly bills) from multiple providers.

This is likely to be a sticking point for all carriers, unless they adopt an ISP-esque "call it even" attitude about passing roaming users around, all have roaming agreements with one another, or all saturate all areas with their own network services.

If carriers can't come to terms on settlement, perhaps the space will be ruled by service aggregators who already handle settlement and billing for such situations.

And, finally, will carriers be able to create service packages for enterprises that enable hand-offs to enterprise Wi-Fi networks while still making themselves financially happy?

It's time for answers
These questions have been kicking around for a long time, but it's getting close to the time they need answering. The back-end business and settlement issues regarding network roaming and integration are biggees and will likely be much more difficult to agree upon - let alone implement - than the technical challenges.