“When I was just as far as I could walk
From here to-day,
There was an hour
All still
When leaning with my head against a flower
I heard you talk.
Don't say I didn't, for I heard you say––
You spoke from that flower on the window sill––
Do you remember what it was you said?”
Who is the  “you”  that Frost is referring to? Perhaps he is attempting to converse with God, seeing that he is using a flower as a form of communication. 
 Throughout this poem, Frost is absolutely certain that God has spoken to him, but he does not recall the exact words.
The Telephone