A solid sales voicemail can't be too intrusive or read like a list of demands. At no point should you say something like, "You need to call me back," or sound frustrated by the fact that they haven't gotten in touch with you.
Go to the website of AceThinker Online Audio Recorder, and click on the "Start Recording" button from its main page. For starters, click on the "Open Apowersoft Online Launcher" button to launch the audio recorder. Next, plug one end of your audio cable to your computer's microphone jack and the other end to your phone's audio jack to connect both devices.

"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!"
Obviously, I need to update it. And if you haven't changed your voicemail greeting in over a year, you're likely in the same boat.