My husbands babymama won't let the kids call him?

My husband and his ex went to court to establish a parenting guidline when he got joint custody. He was dragged into court for a child support review to try to get more money, and finally my husband has gotten something in writing to mandate when we get the kids. Now the problem is the mother won't let the kids call their dad except for on Sundays between certain hours so that she can monitor the phone calls. The kids are 13 & 14 and have cell phones which she threatened to take away if they text or call their father outside of the Sunday hours. Can something be done about this? My husband is torn up over this as are his kids. They spent Sunday crying to him about how awful it was. HELP