Looks like beside the previously announced BT Fusion, there are already other FMC (fixed mobile convergence) solutions in some local markets. For instance OptiMobile UniPhone.
OptiMobile UniPhone for dual network capable devices (for instance supported Windows Mobile Pocket PC Phone Editions) is reality already by completely integrating all telephony services in one client which relieves users from issues regarding selections and differences between cellular and W-LAN networks for telephony. Any call at any time is seamlessly routed over the most optimal available carrier of the call regardless whether it is over the corporate WiFi (W-LAN) network, cellular or other available WiFi network such as home based access points or HotSpots.
Through WBX Enabled Services users are able to reap the full benefit of OptiMobile's WBX technology. Subscribers of WBX Enabled Services are completely shielded from choice of network, i.e. W-LAN or mobile network. Using a terminal equipped with WBX UniPhone, the WBX Enabled Services provide users fully seamless use of WiFi and cellular networks for all voice calls. When users are situated within W-LAN coverage (e.g. within an enterprise, W-LAN public access or at home) calls are made using VoIP. The WBX Enabled Services provide guaranteed seamless handover of ongoing voice calls between WiFi and the cellular networks, thereby offering the users full mobility without loosing or interrupting important conversations when moving in and out of W-LAN coverage.
Currently supported Windows Mobile Pocket PC Phone Editions are HTC's Blue Angel and Alpine as well as HP's iPAQ h6040.
As it looks, there is no client you can simply download and install but this is a complete service which needs to be integrated into the backend as well. At least I haven't found anything but you can purchase mobile terminals equipped with WBX UniPhone through OptiMobile's selected partners OptiMobile's partners.
Seems to be another interesting concept and as far as I have understood it, it's already available in Scandinavian countries.
Last but not least, another FMC solution I have to mention here is O2 Germany's Genion concept which is another way of FMC. While it doesn't use W-LAN but GSM only, it's a zone-dependent tariff plan (in your Homezone you do voice call from your mobile phone for fixed line tariffs) and also includes a separate fixed line number which is linked to your mobile phone.
I might explain it more detailed tomorrow.
Cheers ~ Arne