VoIP and Wi-Fi were never designed to work with each other, but the strong demand for it has resulted in an arranged marriage of children. Both technology are barely out of their infancy and just beginning to enter in to the main stream of everyday life, but there are serious obstacles to the success of this marriage. There has been much hype about the pair replacing conventional cell phone technology and that day will come but, just not any time soon. For now, the technology has a niche in Vertical markets with highly controlled LAN based Wi-Fi environments and limited use among road warriors when they've settled down long enough in a remote hotspot. One thing for sure is that you won't be seeing someone whip out their PDA at the taxi stand for a quick and casual phone call.
Wireless VoIP is a very complex and broad topic that is all to often painted as a single issue but it is not. Wireless computing can encompass everything from Wireless WAN technology to 802.11 Wi-Fi but let's stick to Wi-Fi for now since there isn't even a pretense that VoIP will work over a Wireless WAN link. Even Wi-Fi networks have their fair share of differences between the extreme of the hotspots and switched access point environments with the former being least likely to succeed and the latter being the most likely to succeed. Why is this the case? Because VoIP is one of the biggest sissies as far as tolerance to any kind of data stream disruption is concerned. VoIP packet data cannot ever be delayed by more than 1/5 of a second. Anything beyond 1/5 of a second can result in the loss of quality of audio or result in a complete loss of communications.