I still have my landline for 911 purposes. Your cell phone doesn't come up with an address yet (at least I haven't heard that they've fixed this) so if you can't speak the police have to fax a supoenna to the cell provider to get the address which takes several minutes. Those minutes might make a difference between life and death.
I always tell people on cell phones too that if they call 911 on their cell to give their location first in case they can't get anything else out. Then help will know where to go. It takes even longer to triangulate cell towers to find you.
Of course the newer cell phones are coming with GPS already in them but I don't know if a supoenna is needed for that or not.