How old are the children? By law, there is an age (I think age 12) when the child can determine to live with whomever then just put that request through the courts. Otherwise, all you can do is document and try to have the case re-evaluated by a judge. Unfortunately, its a civil and familial issue; the mother has the right to refuse access to the children to the father on her custodial days. It's not right but they can't to much about it. The police can't do much either and will tell you to handle it through the courts.
In my case, I have given up on calling my kids on their custodial days with their grandparents (Daddy is rarely in the picture) because I would find out later they would get punished when I called. I also tried calling CPS, but they felt due to it being a custodial issue, I was being vindictive with the report rather than being a concerned parent. Just like their diet and my other concerns, I only have so much control pertaining to my kids when they are over there. So I make the best of it when I have my kids at home; they have two entirely different household environments. I have no qualms letting them speak to their father or grandparents (though we are barely civil to each other) and I have strict rules and expectations including homework, reading, manners, and chores, and eat healthy. Over there, I swear its like a free for all. I have learned, for the sake of the kids, to choose my battles, otherwise, its them who get hurt in the long run.
In your case, due to the fact that you two are so far away, I would suggest having the courts reestablish new visitation guidelines so you can see them more often, but don't expect the mom to be forthcoming or cooperative on any level, or changing her habits of allowing the kids access on her custodial days. Seems shes spiteful like that rather than thinking about the well being of her kids and their relationship with their father. Also, the issue may be that she has a problem with you and she fears you will replace her as their mother; a viable fear which then results in her acting out with you and the kids.
I understand your frustration and good luck.
A.