"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!"
Recording Your Greetings Press the Message key on your Grandstream phone or press * 97 from the phone Enter your password. In default, this your extension number, followed by the # key. Press 0 for Mailbox Options. You have the option of recording up to three greetings.

You can also upload an unavailable greeting using the OnSIP app. After logging in, click on the Voicemail Settings gear icon at the far right. Make sure that the Custom Greeting is toggled to "on." You will then see an option to upload a .wav file of your choice. This recording will serve as your unavailable message.
In the U.K. and most of Europe it is considered good manners to leave a slightly longer answering message as it gives the person time to adjust to the fact that they've been put through to an answering machine, compose a message in their head and then get ready to speak. It also leaves the caller with the impression of someone who is calm, composed, polite and not rushing around like a chicken without a head.