15 year old girls! help! how do i calm one down?

she always gets angry at me randomly over msn, i have tried ringing her to ask her directly what the problem is, which worked for a while, but now it doesnt. i tried just agreeing with everything she says, that doesnt work. tonight i tried to completly ignore her and just say "im not talking to you while you are like this", that just made things worse. i need help kinda fast cos she is probably just gonna finnish me soon and i dont know what i can do to just chill her out when she gets angry. its not always randomly, sometimes she miss-understands what i am saying, and i try to tell her that isnt what i meant and she just ignores the fact that im trying to explain what i meant. she is 15 if that helps??