When do you call the cops if a child is missing?

Update! She is home. Her phone is dead and she decided to hang out with friends without telling us.

Here was what I was going to ask since I think it would be good to know:

My daughter should be home by now. She’s fourteen. She takes the subway to school and back. She had a half day today and isn’t home at 4pm when she usually gets home. Her phone seems to be off or dead. She always calls when she gets home. What should I do?

What have you done in similar situations?