The reason seems to be that she has been rather cruelly, according to her, dumped by Mark. And so, over a period of weeks she calls and calls. One assumes that in between spates of calling, Mark actually calls back, they talk, meet, something, and then he dumps her again and she starts calling again. In some cases she leaves as many as 15 messages in four hours. Some messages are only three of four minutes apart.
Sexual content is all but missing.
Sex, which lurks like a Cold War spy hunched uncomfortably against the damp wall of this one-sided conversation, is almost never mentioned at all. I'm not kidding. While it is obvious that the woman is in real emotional pain, and the boy is a classic American dolt who'd really rather just have his Miller Time alone with his bros in a No Chicks Allowed tree house, what we never really know or understand is what these two ever had in common in the first place. In a way, this could be viewed as an anthropological study, a cultural artifact. This is what we are now, in the early Twenty-first Century.