In my experience, genuinely wanting poly and believing wholeheartedly in the ideology of it doesn't mean that all discomforts just get switched off immediately. In my case, I'm the one who knew I was poly for years and tried to steer my husband in that direction, and now that we're finally there we both feel like it was the best decision we ever made for our marriage. And yet, even as the one who encouraged it, it's still a process to be totally comfortable with hearing about him and other women after all these years. I don't feel jealousy, and I do genuinely feel excited and happy for him. And yet, there are still insecurities and discomforts, too. I think that's really human. Even if our hangups about sharing our partners are all just a result of social conditioning, they're still pretty deeply ingrained in us, and we can't always just shake them off overnight. I think the important thing is to not beat yourself up for how you feel, allow yourself to have those feelings and work through them. And personally I think it's always fair to let your girlfriend know that you want less detail at this particular point, and that you're not comfortable with her doing something like picking up the phone in the middle of sex. I like to know who my partners are seeing, and it's important to me that they're totally willing to give me however much detail I ask for, but at the same time I'm really not in a place where I'm comfortable hearing too much sexual detail just yet. I'm a bit insecure physically and sexually, and while I don't at all fear losing either of my partners to someone else, I still have a difficult time not feeling inadequate when I actually imagine them naked with someone else. I have to imagine that's not terribly uncommon.
I don't think you should necessarily doubt your ability to be happily poly just because you're working through these discomforts right now. Don't be afraid to tell your girlfriend what pace is comfortable for you when it comes to hearing about sex, and keep rereading all the reassuring things here! What you're experiencing sounds pretty normal to me. Good luck!