The set-up is cruel - why would you expect an 11 year old to make the phone call? That responsibility should be on the adult. In our sitch, each parent calls the kids at about 7 each evening. I even ask the kids if I am interrupting (if they are busy I just say love you and pretty dreams so they can get back to their life). X is more demanding about speaking to them at length during "his" time, no matter what they may be involved in.
Maybe you can tell your son that he can make that suggestion to his mother, as a way to address and resolve the problem...?
