M.C.
Hello E.,
It is very normal for kids to go through an imaginary friend stage. Although, as a teacher, it is a little concerning that she will not respond to you when you call her. Not to scare you, talk with her pediatrician, or perhaps have her evaluated by a child study center, if they can do that, and see what might could be behind these imaginary times.
I hope this helps, and I hope I am not way off.
M.