[BBM]
I've come to believe it is Ron covering for Misty. The motive? He told her to give Haleigh something that night to make her sleep and it was Ron's illegal drugs that she gave her. Ron knows he's complicit in the crime.
As to why Misty didn't pick up the phone? I think she was busy with the task at hand. Perhaps she went with whoever was getting rid of the evidence and left her phone at the mobile home. This girl isn't exactly the type to plan ahead.
I don't think Misty will ever break. She can't because she is the one who gave something to Haleigh. Ron can't break because he's the one who told her to give Haleigh something and it was an illegal drug.
For those who think a druggie wouldn't waste their precious stash on a child, I disagree. As a rule they wouldn't, but a one time, I'm pretty desperate here, I need sleep and this child won't go to bed type thing? Yeah. It would also depend on the quantity of their supply at the time. If they had enough on hand, yeah, I think they would give her something. I think Misty maybe mistakenly gave her an Oxy or didn't follow Ron's instructions as to what or how much to give her.
Ron blames Misty and Misty feels obligated to Ron for killing one of his children. Misty doesn't realize that Ron would be at fault also.
That's my take. No motive, pure accident.