Another limited combined Wi-Fi/cellular offering hits the market, this time from DoCoMo: Like the other services introduced to date, this one has its limitations. Users will be able to make voice over Wi-Fi calls but only in their offices and only if their office has a special server from NEC. Voice over Wi-Fi won't be available outside of the office, even on DoCoMo hotspots. It sounds like even data over Wi-Fi will only be available on hotspots built specially for the device.
It's a combination of technical shortcomings and uncertainty about how to make the best of Wi-Fi that is preventing cellular operators from offering seamless combined services. Ultimately, the cellular operators will have to make combined offerings because Wi-Fi is popping up in more places and customers want the high-speed access. Cellular operators may lose some potential data use to Wi-Fi, but realistically, the cellular networks cover so much ground that they'll still get their share of the market. The same goes for voice over Wi-Fi services, which are more of a threat to the local phone companies than the cell phone operators. Voice over Wi-Fi phones won't be terribly useful as mobile phones but they'll be great for the office or the home.
The cellular operators are notoriously slow at picking up new technologies so it would be no surprise if it takes a very long time to see a usefully integrated, full-function combined offering.