Her father's life/death was not dependent on whether she answered the phone or not! Her sister calling her over and over instead of acting on their father's behalf could just as easily be "blamed" if you want to waste time blaming; you are not God to know what would have/could have/did not happen anymore than you can see into the future.

You do not owe another person, other than perhaps your child, anything. Your #1, sole job is to be the best You you can be and that's a full-time job. You cannot be a friend to another with the mindset that you are a friend because you owe them something; the other person does not want or need that! They want/need You, not your misguided idea of how you can make things feel right for yourself. Deciding you owe another is actually selfish; we are our own persons, I am in charge of who I believe owes me, what, not the other person! Has your friend said you owe her and should work to pay what you owe? Of course not!

And since when is love a "problem"?

It sounds like most of your problem is with you and your thoughts and feelings; only you can adjust them. I would maybe find a therapist to talk to and discuss your wanting to judge another for their behavior when not with you. What she did/did not do/experience with her boyfriend are not something you can know anything about, despite her telling you about them; you were not there and do not have but her half of the story. You are reacting to things in other people's lives too much from your own self and you are not "there", it is not your experience.

Your experience says, "if you don't answer the phone and the person on the other end wants to get in touch, they will call again" and that is wonderful, very true (in my experience) experience! Your experience is that the two of you enjoy talking, would rather talk than talk to others on the phone; a very wonderful experience! (Been there, done that).

Neither of you knew who was on the phone or that there was a problem but if you could have, would you have taken that last loving five minutes talking with you from your loved one in order to deal with the difficulty and sadness of her father dying, five minutes earlier? Her father died. Her father had to die; he could not live forever and you cannot determine if he would have died when he did if she had answered the phone sooner or not (because it did not happen that way!)

People mention the problems with "worry", worry being about things that have not happened, contemplation of the future and others are known for living in the past (where you appear stuck trying to reorder what did in fact happen with your version of would have/should have/could have) and that only shows how important it is to live in the present only. That's all we have, our own present.