the ex is mad again GRR!

My ex (my children's father) is mad again! This time he is mad because the kids don't want to talk to him when he calls. He doesn't understand why they don't want to talk to him and why I don't make them?
I tried to text him Saturday and tell him that we were going to be at the fair/rodeo that evening and that he should call earlier in the day to talk to the kids. He says he never got the text and called at 7pm like normal. We had just finished the rodeo and were getting ready to head into the fair. I took the kids to a quiet place so they could talk and told them that we wouldn't do anything until they were done talking. Of course the kids were excited about the fair and only wanted to say hi and I love you. I tried to get them to talk about the rodeo and extend the conversation but my son hung up the phone.
20Mins later I started getting texts saying that I need to tell him when we are going to be busy so he can call early. I told him I tried but we were not going wait around all day for him to call. My son was competing in the rodeo that night and we had to be there on time.
The kids on more than one occasion have said they don't want to talk to him and I try to encourage them to talk to him as much as I can. I do leave it up to them how long their conversations go and if they don't want to talk, they have to tell him. I encourage them, one because we live 1200miles from their father and when they are with him for the summer I don't want him to try and play childish games when its my time to talk to them.
Most of their conversations consist of him talking about how great his life is and his new dog. The kids try to talk about their day but he always brings the conversation back to himself. He puts down the things they find fun and interesting. They like to watch WWE wrestling and he tells them that its fake and stupid. He has never taken an interest in what they like and doesn't understand why they don't want to talk. I've tried to talk to him about it but he says I'm just trying to parent him.
At the end of our text conversation on Saturday he said "the door swings both ways and I better remember that this summer!"
I told you all this so I can ask for some advice. How do I encourage the kids to talk and get him to understand that if he doesn't take an interest in their lives, he's just going to push them away more and they will never want to talk or come to his house for the summer?