Ok, first of all, caller ID is cool to have but it doesn't really help with the un-realism behind not actually -hearing- the voice on the other end of the phone. Just knowing whose phone is being used to call you doesn't tell you who's using that phone.
I have to point out that not only would you actually be able to recognize whether the voice is male or female, you would also detect any accent they have. I know a player who always used to put *english accent* before the first thing he said in a phone conversation. That was a neat touch.
We could all just start putting *a pleasant female voice* or some such descriptive thing before what we say in the phone. That's one way to RP it. Or, if your character is calling her boyfriend and he'd recognize her voice, you could even put *Tylissa's voice* but of course that still isn't as perfect as it'd be if we could really see a coded difference.
Yet again, one neat idea that may happen eventually. In the meantime, let's keep calling each other and having fun! =P