If the user holds the handset to the top of the head, the back of the head, cheekbone or jaw and plugs his or her left ear, the call will be heard internally on the left side.
Quote from the article at I4U.com
“The Tu-Ka group has launched a major advertising campaign for the new cellphone, featuring a young woman and a X-ray image of her skull using the handset.
The cellphone is priced at 7,800 yen (73 dollars) each at the discount store. Tu-Ka firms belong to Japan’s second largest telecom carrier, KDDI group. “
