You have arrived at a new phase in your divorce: a power struggle between you and your ex's new girlfriend, and between her and the children, for the support and favours of your ex. The strongest person sets the agenda. If your husband is deeply in love with her, then it will not be easy for him to set limits, and he will put her needs before the children. Women have effective tools for getting their way with men.
You are worried, because your ex's new girlfriend does not see things the way you would like her to. She, on the other hand, sees you as a threat and a problem. She may want children of her own. She may not want your children to inherit from him to the detriment of hers. She may prefer to have your ex to herself without the children. She certainly does not want you and your ex to have a close relation. Every time you talk to your ex about the children, she will be upset. If the children can call him whenever they want, she will be uneasy and never be left in peace with her beloved. If it were possible, she would prefer to spirit away you and the children.
What can you do? You will have to accept that the balance of power has changed, and that the welfare of your children will not any more be of highest priority to him. In this new situation, you have to be sensible and mature. Maybe the new woman would feel less threatened and excluded if you talked to her, rather than to your ex. Even when you talk to your ex, she may be in the background directing the communication. If you talk directly to her, she may feel more secure and in control. In such talks to her, you have to be supportive and understanding, not critical and accusing.
Would it be possible for your ex and the children to call you at times when she is occupied with something else, such as showering or shopping? Maybe she would not be so disturbed by such phone calls if she is not directly confronted by them?