It is not YOUR responsibilty to call and see where he is at when HE is late. HE is responsible for his actions and HE knows that he has to be on time to pick the boys up to take them to school. You did the right thing in hanging up on him.
You have already advised him that if he was late, you would take them to school. You set the boundary and he definitely missed the mark. Stick to it and do not waffle. Bottom line...he needs to face the consequences of his actions.
I'm glad you hung up on him. He should have called you to tell you where he was at since HE was the one running late. Sometimes, they have to learn the hard way and I guess he's going to learn to set that clock a bit earlier and get a move on. LOL!
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.