C.C. isn't actually a witch, far as I know. She calls herself that, probably to reinforce her belief she's not really human. Something like 600 years ago, she was just a human girl who got her own geass (it compelled people to love her (but only temporarily)). But the woman who gave it to her was immortal and wanted to die, so she forced her immortality on C.C. The only power C.C. has is to grant geass contracts with people, and not die. I'm not sure how she'd ping to Sebastian. :|a!!