That being said, you want to stop short of coming off as aggressive. Remember to keep your tone conversational, and try not to badger prospects with too many voicemails. There's a middle ground between being pushy and being a pushover — try to strike that balance.
When you hear the Dial Tone, call into your landline or office voicemail box. The rule of thumb is when you hear the Voicemails Forever dial tone, then follow your phone's instructions to listen to your voicemails remotely. This will vary from carrier to carrier and phone to phone. You may need to call your phone networks customer support for updated instructions.

Additionally, you will want to close out your voicemail by (re)stating your contact information and name and purpose of your call. This is another little trick of leaving voicemails - people tend to remember the first and last things you say more so than the middle, so by summarizing the important pieces of your message at the end, what you said is more likely to stick.
"Dear [Prospect's Name], this is [Your Name]. On [Date of Last Contact], I contacted you about [Reason for Previous Call] and as I haven’t heard from you, I can only assume you're either, one, not interested and I’m simply clogging up your voicemail with annoying spam messages; or, two, you desperately want to contact me, but you’re trapped under a fallen filing cabinet and can’t reach your phone. Please let me know which one it is. I'll gladly send help if it's the latter. Just call me back at [Your Phone Number]. Thanks for your time!"