Can an ex refuse to let you talk to your children via telephone (live in different states)?

My husband has called his kids almost every day for over two months and not been allowed to talk to them (they are 4 and 5). His divorce decree says he gets them every other weekend. Two months ago he moved to a different state across country shortly after his divorce was final. His wife is stating that he if he wants to talk to them he is welcome to come and get them every other weekend, but otherwise she will not let him talk to them. She has stated that they don''t think about him anymore and they are trying to forget about him. She has told the kids that he left because he doesn''t love the family anymore. He needs to talk to them so they know he still loves them. What are our options? Can his weekend visitations be enforced as phone visitations, or do we need to get a different order for this? If so can we apply for it in Minnesota (where we live) or do we have to file it in Texas (where the kids live and where the divorce decree was finalized)?