Let's consider how TeleSym's solution works in a public Internet access hot spot:
How well this actually works depends on many factors, ranging from the quality of the microphone on the PDA to utilization of the hot spot WLAN to congestion on the public Internet. If you want to try this yourself, TeleSym makes SymPhone client software available for free 30-day trial, routing calls through their own call server, PBX connector, and 3COM NBX in Seattle.
Note that this usage model is different than simply carrying a cellphone. Hot spot coverage areas are small (hundreds of feet, not miles), interaction is required to connect to each hot spot, quality and usability will vary, and outside calls go through your own PBX, not directly to/from a local public carrier network.
Cellphone manufacturers like Motorola and Qualcomm have announced plans to build Wi-Fi enabled phones and chipsets. Once that happens, carriers may offer seamless roaming between 802.11 and "3G" networks like GPRS. For example, a carrier like T-Mobile might offer a plan that lets dual-mode phones use 802.11 at hotspots and GPRS everywhere else. In that case, the carrier would track phone location, route incoming calls from the PSTN, allow VoIP protocols into the hotspot, and automate device authentication. In return for providing this additional infrastructure, the carrier will charge you for the call, of course.