Cell phones offer countless bells and whistles, but South Korea's SK Telecom offers one of the most unique: a ring tone said to repel mosquitoes. For about $250, customers can download a sound file that is inaudible to human ears but not to bugs. The company says in tests it disturbed mosquitoes within a three-foot range. Customers play the sound by pressing buttons on their mobile phones. Generating the tone drains phone batteries as would a regular ring tone-but users may prefer that to mosquitoes draining blood.