Does she have any kids? If not, she won't understand you at all.....
I don't have kids and I was in a relationship with a guy with three kids....oh boy....I could not understand it at all....I was forcing myself to be nice and understandable....but I wanted him all to myself....
Well...I ended up leaving him, because I thought his kids are more important than me and I don't have to interfere with his parental life....
I was the one who couldn't understand and I left....of course, it was a drama but I'm glad I did that....
I think your girlfriend doesn't have any rights to tell you where to put your cellphone....and I think she's just sensitive about the situation....that's all...
Be firm with her and tell her that your kid is important to you....