So um, why do you think your fiancee wants to talk to her, and why the hell does she need your permission to do so?
You, your ex, and your fiancee should ALL be on talking terms. If this is impossible, then you all need a neutral third party to help you figure out where to meet to drop off the kids w/each other, resolve child rearing issues and ensure good parenting in both houses.
Unless of course you are doing something wrong to your ex and your children that you want to hide from your fiancee.