This thread cracks me up. Seriously, I couldn't stop laughing at the first bunch of posts.
I'm not Chinese, but when one of my really fobby honkie friends calls I'll go "weeeeeeeeeeeeeeiiiiiiiiiiiiiiiiiiiiii" at them. If it's an unknown call I'll just answer with my businss voice ("hello", "yes" or [my name]) and if it's someone I do know there's often a private joke ("yello? IVAN!" or "tora tora tora desu!"). With family it's "¿hola?".
Oh, and sometimes it's just "mm?" but sometimes it's the person's name (people STILL don't know how I can tell they're calling...
)