I think it’s rude. She should have told the person on the phone that someone was at the door. Then she should have let you in and she should have told you that she was on the phone and she’d just be a minute. Then she should have just been a minute.
Unfortunately, people these days, because they can be reached 24 hours a day, think that they have to be available 24 hours a day too, but they don’t think about the etiquette involved in making other people wait, or making people have to listen to them shout conversations in public areas, or the danger of making them have to dodge their unsafe driving while they’re talking or texting in their cars while they’re supposed to be driving.
If I were you, I probably wouldn’t have called her out directly, because she is your friend and it most likely wasn’t her intention to be rude (although she was) so I might have just said something like, “Is your brother OK? Maybe I should come back another time.” which might have prompted her to say something like “I’m sorry, no it’s fine, come on in. Sorry to have kept you waiting.” But I wouldn’t have counted on her saying any of that. I’ve noticed that when people are engrossed in phone conversations, they simply don’t see/hear/think about much else. It’s a shame.