There are many easter eggs to be found in Tattletail.
Knock on Mom's Door
Nothing happens for now when you knock on Mom's Door when Mama Tattletail is not after you. But, when Mama is chasing you and you manage to knock on Mom's Door, Mama will jumpscare you and you will get an achievement.
Phone Call
When answering the phone, a strange voice talks to you, it appears to be the announcement of TattleS (December 25)
Secret Ending
When you don't answer the phone call, collect all 22 eggs, and start opening your presents, instead of getting the Mama jumpscare, you receive special gifts from Tattletail. These special gifts include Tattletail's tag, and a Waygetter golden flashlight. And, when you knock on Mom's door, you are greeted by your Mom.