Aren't some airlines carrying battery fire containment bags?
Example (not an endorsement in any way - use at own risk)
Lithium battery safety bag for thermal runaway fires - LithiumSafe
I would not want to use an extinguisher on a battery fire - problem one is that the source of ignition energy is inside the phone as is the organic electrolyte that can fuel the fire. Use one, class D, preferred, if you have to or to extinguish surrounding materials. As long as the lithium ion cell has charge it can re-start and it might be some time after it seems safe, so an extinguisher alone is insufficient protection.
Water submersion is also good, but of course it's on fire and will likely severely injure anyone just grabbing it bare handed. Using water ensures the cells remain below the ignition temperature and keeps air away from the ignition source.
There is no free lithium worth mentioning in lithium ion cells so the aggressive metal fires that water makes worse aren't a concern. This danger is the case for primary lithium cells (non-rechargeable/single use). Those will react badly if ruptured and made wet.