It is generally inappropriate to use a character's name in the character cue BEFORE you've introduced the character even if some other character says the character's name unless you cut to a shot of said character in order to introduce him or her.
Of course if it's your own script and you're shooting it yourself, you can do whatever you want... I'm talking about a spec script or a script you're attempting to obtain financing for. Inevitably, someone down the line is going to have someone who knows what they're doing, read the script and if in reading the script, someone who knows what they're doing finds a name without the appropriate introduction beforehand, they will most likely think they missed something and flip pages back to find the intro.
And... When it's not there, then your script comes off that much more unprofessional.
Generally speaking, you simply give the character some kind of temporary name as you've already pointed out above when we haven't yet SEEN this character.
I have seen character introductions for characters we've only heard on the phone but not yet seen as outlined above but they generally come off fairly clunky.
In the very first post in this thread, the correct way to accomplish what we're discussing would be to cut to a shot of the character during the phone conversation and write his introduction to the story/script.
However, this ALL DEPENDS on your story... In the same example above, there might be a very good reason the writer doesn't want us to see John's brother... What if he's in a wheelchair? What if he's a transvestite? What if he's in prison? In a case like this, the writer might not want to reveal this information till later on in the script. If so, then the way it's written would be appropriate.
But generally speaking, you don't use the character's real name in the character cue until we actually meet the character through the written introduction of said character. Before that, using something temporary like CALLER, VOICE ON PHONE, etc. is fine.
filmy