I might be shot for what I am about to say but I wouldn''t waste time in asking for
mediation at this stage.
The ex''s way of handling what is ahppening is very disturbing and when a parent disconnects phone, doesn''t asnwer them or instructs children to not answer their home phone ''even if it is daddy'', then you understand that things are becoming very serious.
As I said before, I do understand the reluctance and fear of going back to court with this but if you do not act fast the more time the child stays away from her father, the more difficult it becomes to re-establish normal contact. Also, when it eventually reaches the court it might be used against your partner.
My partner''s ex stopped overnight stays herself and when they both appeared in court she started claiming that now it will be soooo difficult for the children to go with their father as they haven''t seen him in four months. Thanfully the Judge was totally intolerant and told her that this is exactly what she is taking care of right now. The children I am referring to though are younger.
Good luck to all of you.