What was so vital about retrieving his mobile phone that he would endanger his life? Even if he'd got it back it would have been unusable, and if he'd left it, presumably it would have been entombed in the (very watery) concrete.
I've still not worked out the significance of Phelan noting down the number plate of the driver of the car/van that tried to pick Summer up after school when she missed the bus. I was expecting another story to develop there.
I thought he went to try and get his mobile because it fell in with the body/ies (not sure if he put both in? and the mobile would of pointed the finger at him? maybe im wrong, but did he even get it back?