No, you don't HAVE to make them talk to Dad, you WANT to make them talk to Dad. Parents and adults decide about these things because children are too immature to make wise decisions even in their own self-interests. When parenting apart the most loving thing the custodial parent can do for the child is facilitate a healthy and positive relationship with the other parent. Children need and deserve to grow up knowing both parents. Swallow hard and step up to the plate with some enthusiasm. Help the child with things to talk about. Teach your child a song to sing or poem to recite. Try Skyping so there is a visual component. Someday your child will thank you.
Best wishes for an outcome you can accept, and please remember to designate a best answer.
This answer is offered as a public service for general information only and may not be relied upon as legal advice.
You should encourage the children to speak with their father. I understand from an attorney standpoint and a father who had a similar issue, and my youngest never wanted to speak on the phone and after several discussions with my son, he actually encouraged her to speak with me. You did not indicate their ages, but I totally agree with the last attorneys answer. You should encourage them to speak with him, if even for a brief time, so Dad can at least let them know they are missed and he loves them.
There is no clear answer in MO. All parents should do their best to get along , but it doesn't necessarily work. How old the kids are is important . Kids under 8 don't have a very long attention span for phone calls. My grandkids 7,8 and 10 don't talk very long to me . The older one has more to say, so age is important. Document the kids feelings to dad and ask him to do what is necessary to improve the relationship. Maybe skype would work, but he has to understand that age will govern how long they will stay interested in the conversation. Have dad make a phone call , maybe at set times ( 2 a week ) to discuss things with kids. Do your best to have them available and ready to share with dad.