Firstly you can follow this fantastic guide by Gage on how to get to the point I will describe
HERENow you want to wait until you are in Cavity City and have opened the back door to meet Slappy the dummy.
Once here keep selecting your cell phone from your inventory and try to call your brother over and over until the phones icon is an empty red battery symbol (As seen in the video). Try to call again to make sure that the phone is definitely dead.
Now press
to meet Slappy and do the following:
• Select dummy on left
• Select dialogue "Don’t Give Name"
• Select dialogue "Stammer it Out"
• Select dialogue "Sincere"
• Select dialogue "Whatever"
• Select dialogue "Challenge"
• Select dialogue "Scrutinize"
• Select dialogue "Shrug"
• Select dialogue "Made Ya Look"
• Select dialogue "Fight Back"
And the achievement will pop once Slappy is back in the book.
Here's a video of the final scene with the phone dead.