I totally get where you are coming from, and can see both sides, and I experience both sides with my DF and his boy. However only you know your kids and ex, and what they are like. I would suggest, that because he is taking them on holiday and you are allowing this exception, then I think he should adhere to your wishes to an extent. I would try and negotiate this phone call thing. Tell him you understand that you don't want to interrupt his holiday. But could you please call on Monday, Thursday and Sunday at 730pm after dinner and before bed. Or perhaps when they go to bed, so that he won't have any ramifications of them being upset and having to deal with it during the day. Maybe try and compromise with him somehow ? I know it I hard and sucks