I think you should be able to expect that he tells you his plans with her before his time with her.. Him not telling you anything until you call and hear her in the back ground is a lie by omission. You don't have to tolerate that..
My hubby has a secondary that he happens to work with. So in my mind h=they see one another all day. They get the office smiles, and texts, and flirting, and I feel like the harried house wife. He continues to reassure me that my romantic ideas of what life is like in the office daily is just that "my romantic idea". He also reassures me that I am not simply the house wife, but the operator if a finally tuned child rearing machine and that I am not just a harried house frau.. which at times is how I see myself compared to the office sex kitten..lol Thank goodness for his reassurances...
Recently I suggested that the two of us become friends because I tried not being friends with her and that wasn't helping me any. I text her and she didn't respond, then lied and told my hubby that she hadn't received a text. Shortly after that she emailed me and said that she had received the text but had not noticed it in her text thread. So I made my attempt at being friends, and shortly discovered that it's not worth being that friendly if she won't be honest. So I happily dropped the idea of being friends and went on about my business. I am happy for you that you can at least be friendly..
As I read your posts, I could totally relate to what your going through and feeling.. Writing might help get your thoughts and feelings more in order too..
I would definitely continue to keep communicating with him and let him know what your comfortable with and what your expectations of him and her are..