Hi, I upgraded to ML recently and I used iMessage to contact my sister. Everything was smooth, then a couple of weeks ago I wanted to talk to her and it kept on saying that "she is not registered". I looked around it and though that maybe i wasn't signed in correctly, so i signed out and back in. This kept on showing up, i tried to contact applecare but my iPhone kept on giving me "Call Failed" I tried to find a solution on apple community and they were of big help but it didnt work. i remembered that i tried my other sister's ID to sign in and it signed in beautifully, i asked my sister to sign me in with her mac and it did. This means the problem is not my ID its my mac. my Q is, can i delete iMessage and then re-install it? Or can i delete it but re-install ML again?