A relatively unprofessional one — like mine, for instance — does the opposite: It encourages prospects, recruiters, and potential connections to run in the other direction.
To end the voicemail, state (or restate) your main question, then say your phone number. You can also say you’ll follow-up with an email. Now, this isn’t the same as saying, “If I don’t hear from you, you’ll hear from me!” Instead, it’s giving them another way to contact you, one they may be more comfortable with. They should still feel a responsibility to respond to your question, but now they have two options for how to get back in touch. (Then send that email, of course.)

“If I leave a voicemail message on my first call in the morning, and I get voicemail again on my second attempt (either later that day or the next time I try and reach them) what do I say when that happens?”
The tone of your voice when leaving sales voicemails is almost as important as the actual words you say. If you obviously sound like a salesman, your message will be deleted before it even ends. You want to speak in a tone of voice thats friendly, confident, and casual—not overly excited, nervous, or desperate-sounding.