When calling someone to ask to speak to them, and you find they are not available, the immediate decision is to leave your name, a brief description of what you wish to talk about, and even more importantly a working telephone number. :-)
The problem with not doing this is that you are going to be left in a situation of whether, indeed, the person you were trying to call can't actually call you back because you hadn't given adequate information, or if they don't want to speak to you based upon your brief description.. possibly even, if they are really weird, they don't want to speak to you based upon your name. Saying that, they wouldn't be so weird if you name was "son of the devil", "sh*t faced charley" or something to that effect.