For the divorced wives- is it impossible to have a relationship with the new wife?

I am not asking to be her best friend, but why can't this woman at least give me a level of respect? Her children adore me. They come to me with everything. I married him so now I am a permanent fixture in the kids' lives. When my husband talks to his ex all they do is scream and shout. It always turns into a personal battle and an ugly name calling session, rather than a productive phone call about the kids. So last night, same ugly fight ensued between them and it ended with my husband slamming the phone down and yelling at me- "You talk to her!" Since it was a topic about the kids and I am a relatively reasonable person I figured I would give it a shot. She refused to take my call and later texted my husband to not have "that woman call me again." So, now I am the angry one. I have always tried to treat her with respect and give her the benefit of the doubt in many situations. After all, I know her ex husband as well or better than she and he can be very tough to live with! So, I could almost sympathize with her at times- but not anymore! We have been together for 5 years now. I think she needs to get over it and accept/appreciate the fact that her children have a person in their lives who loves them and cares about their well-being. So, my question to you divorced women- is it impossible to have a workable relationship with the 2nd wife-- especially when there is a serious lack of communication between you and the ex husband? Am I expecting too much to be able to act as the mediator so good decisions can be made about the kids without every phone call turning into world war 3?