Not as if you are talking to a second grader, but as a matter of courtesy to make sure the recipient is able to write down the key information from the message. This raises the perceived level of importance attached to returning your call. Another quick and easy response to the Guardian of the Gate who wants to take a message is to ask whether the manager has voicemail. If so, asked to be put through to it. You dictate what goes into your message rather than the scribe at the other end of the line.
Voicemail is always better than a scribbled note and has greater professional obligation for action.
If you get no response to your initial voicemail message after at least three days, call again and leave a more detailed message. Give a quick synopsis of who you are and what you can provide to a potential employer. Ask for a return phone call to further discuss the employer's needs. And keep trying until you do get through. And please, please, please remember that you definitely need an answering machine to field calls when you are not in.