Normal fires need oxygen in the surrounding environment. The use of Halon or other suppressants in extinguishers suffocates the fire, removing the oxygen.
Lithium fires generate their own oxygen. Covering them with a halon or other suppressant doesn't work as it's not isolating the fire from the oxygen.
The only way to extinguish the fire is to reduce the temperature below that which the lithium is generating its own oxygen. Obviously water or other similar liquids is the best or that. Ice is NOT recommended.
We have these on board.
http://www.aircareinternational.com/...gement-system/
Haven't had to use it yet. Touch wood.
As for the danger, I'd rather have the phone burning in the cabin, than in the hold.