Radiation from cell phones
The hypothesis that radiation emitted by a cell phone emits enough energy to increase the temperature of water in a test tube is proven to be incorrect.
Many studies have been done to determine if radiation from a cell phone can cause health problems. Most of the studies completed were able to show that using the cell phone does not cause cancer or other health problems. However, it is still advisable to minimize exposure to cell phone radiation by using hands free devices, earphones or Bluetooth devices.
Try repeating the science fair project by using other cell phone models and brands.
The experiment can also be done to determine if the temperature increases during cell phone conversations (as opposed to simply ringing).
Mobile phone radiation and health - http://en.wikipedia.org/wiki/Mobile_phone_radiation_and_health
Physicist debunks cell phone viral ad - http://www.wired.com/underwire/2008/06/cellphones-cant/
Basic safety requirements