Simply pause enough for your listener to have the opportunity to move the discussion to a close if they choose. You don't need a verbal go-ahead to continue. But if you ramble on for 5 minutes without giving your listener the opportunity to gracefully shut you up, you have not only wasted 300 seconds, you have also potentially alienated a contact. Respect your listener. Give them the benefit of the doubt that they have the intelligence to determine if they can help you.
People love to help if they can.
If you get a voice mail greeting or a secretary who insists on taking a written message, leave your complete 36 second career presentation with enthusiasm. One message is all that is necessary. Don't expect that everyone will call back. In fact, don't expect that anyone will call back. Encourage them in your message to call if they have any opportunities available at their organization for you, or if they have any helpful insight to share about who else to contact.