Bono,
the aircraft crash landed not in the water but in dense jungle some where close to water. Phones ring because while passengers might not be alive however some phones survived.
There would have been a huge fire and a lot of smoke.
Also phones are supposed to be 'off' so the chances of one cell phone being 'on' and surviving the crash are very low. Chances of dozens of them surviving are probably infinitesimal, to say the least.