Well there are ideas out there that are similar to what you have described, however- they are very range limited.
This concept will soon be possible though. With the newer Wifi standards that we are expecting to be ok'd in the next year to 2 years you may see more VOIP type phones that will have various modes of access- Cellular, Wi-fi, and base station (old copper)
The integrated business solutions that my company specializes in have actually included VOIP (voice over IP) into highly mobile PC based systems. This litterally allows us to remove the constraints of the office walls and allows up to get out and be more flexible. All we need is either a Wi-Fi access point, or a cellular connection.
I know that this is a more advanced answer for your simplified solutuion, but it's where business is headed.
Joe M.
Executive Manager
Intuitech
Joe M
Intuitech
"efficient business... wherever,whenever"