Cell phones and cars are unusual driving partners but a new phone from Sharp may actually become a driver's best friend.

The phone is the first with a built-in electronic car key. It's compatible with Nissan's "intelligent key" function, which works with 950,000 Nissan vehicles across various model lines, and was developed by the auto maker with Sharp and Japanese cellular carrier NTT DoCoMo.

The phone is equipped with the same features of a normal Nissan intelligent key.

For example, the driver can easily lock or unlock the car from as far as one meter away with a press of a button. It can also be used to start or stop the car, providing that the cell phone is inside the vehicle. An electromagnetic sensor detects the presence of the phone, allowing the driver to start the engine by simply pushing the ignition button. In addition, one can open compartments such as the trunk by just carrying the phone as the same proximity sensors are at work.

The flip phone is based on Sharp's SH906i model, the only difference being the addition of two buttons in the phone's upper right corner that are used to lock/unlock the vehicle. NTT DoCoMo will provide the mobile service along with an additional security function that allows subscribers to disable the cell phone's "intelligent key" function in case they misplace the phone.

In Japan, cellphones go beyond the usual call, text and photo functions. Features intended to make daily life more convenient are commonplace and phones can be used as train passes or even as a wallet for shopping.

"A lot of people have been requesting integrating the car key into the cell phone," says Keiji Ohhira, a Nissan engineer who was demonstrating the system at the Ceatec electronics show that began Tuesday in Japan. "We took their concern into consideration and contacted Sharp."

At the moment, the companies only have a prototype of the phone but there are plans to commercialize the handset within 2009.