J.C.
At first I thought you meant Sept. 11th and I was like - what??? Sorry, this New Yorker's mind will always go back to that day when I see 9-1-1.
Anyway... I think you should give firm examples. If mommy is ever on the floor and she can't talk to you, call 911. If grandma ever falls and she can't get up, call 911.
And keep reminding her what means she should call. She'll get it.