Fascinating. From Noise From Phone Can Chase Mosquitoes:
South Korea’s largest mobile phone operator said Thursday that it will offer cell phone users a new noise service that it says will repel mosquitoes.SK Telecom Co. said subscribers can pay 3,000 won (US$2.50) to download a sound wave that is inaudible to human ears but annoys mosquitoes within a range of three feet. Customers can then play the sound by hitting a few buttons on their mobile phones.
Of course, it uses up the battery as much as ringing constantly would.