We went through this when DS was in 6th grade. There was a girl who called 5 or 6 times a day, and sometimes she'd have her friends call him, too. Mind you, he didn't even KNOW those friends, either! It really got annoying for everyone in the house, and that, coupled with the fact that there was a baby in the house who would wake up when the phone rang, or we'd be trying to have dinner when the phone rang, etc., it got to be too much. I ended up emailing the girl's mom (I knew not to call because the mom worked a lot and there was no other parent in the house. Thank goodness for the student address book *and* technology). The mom emailed back thanking me for letting her know what was going on, and if it happened again, to let her know. Everything was great for about a week. Then, the calls started again. We got a lot of hang-ups and I went ahead and did the star 69 thing. The girl answered and then tried to deny calling. I told her I was going to contact her mom again. I emailed and got no response, but the calls stopped completely. Guess the 2nd time was a charm.
I know that some of the posters said not to worry, but after all of this had happened, I found out from other moms that the same girl and a group of her friends were calling all of their sons, too. One mom even found a lewd letter to her son from one girl, but never said anything about it to the girl's parents. I told her that if it were my daughter, you'd better believe that I'd want to know so I could set her straight. Anyway..... If the girl's calls are really persistent and to the point where your family life is interrupted, try to contact the parents and then go from there. Hope that helps!