Frankly, your husband needs to take the lead on this one. It should be him, not you, telling his ex when it is or is not appropriate to call. It will also be him, not you, who is the subject of any court action to enforce contact (should that occur). It's his obligation to make sure that the visitation order is followed, and your only authority is that which you derive through him.
You make it sound as if you're in the car for a good half hour on the way to school, and you clearly have a cell phone - is there some reason mom can't talk to the kids while they're sitting in the back seat on the way to school?

