A week with each parent is the norm here in Denmark when parents divorce.
It can be hard on the children, but seems usually to be what they prefer, as the otherwise feel they are being made to chose one parent (the one they spend most time with) over the other.
I'm sure your daughter and her husband thought long and hard before divorcing and agreeing to their child spending an equal amount of time with each parent, so please, don't rock the boat. I'm afraid that the child's parents will feel this is not your business if you say or do something, and unless the child is showing signs of actual distress and not the very natural feeling of missing his mum, I think you are best saying nothing.
I don't think it is quite fair of your daughter to keep phoning the child when he is at his dad's. He isn't really getting a chance to settle down there, is he?