Why won't he answer the phone?

My daughter tried twice tonight to call her Dad on his cellphone...he won't answer his phone or return her messages. The kids just wanted to say hi to him and also to tell him good night...he won't even answer the freaking phone. Yesterday, he made no attempt to contact them to say Hi or anything. My daughter is sad and bummed out...I am pissed. What sucks, is that now she is preparing to spend the next 5 days with him. (Along with her brother) If he pulls te crap of not answering the phone when I call them to say Hi or goodnight, I will no longer play the nice ex-wife!