Dealing with uncooperative ex

We have a written and signed custody agreement which says that I should be able to talk to the kids as long as it doesn't interfere with sleep or school. My ex chooses not to return my calls or texts. He believes I am imposing on their time but as their mom, I want them to know I love them and hear about their day. During especially long stretches (vacations/breaks), I start to worry that the kids are ok as I haven't heard anything at all or received any responses to my texts. Our agreement did require 5 sessions of parenting counselor but he refused to attend. I have told him I will seek going to a counselor for this and many other issues (missed homework, refusing to take kids to activities, etc.) but again he refuses to show up so I have not scheduled. I need to know what I can do.