Users often don’t invest enough time into their messages, resulting in incomplete, unprofessional, or otherwise under-whelming greetings. Sure, crafting a greeting doesn’t sound all that complicated; however, there are a number of pitfalls users can fall into—i.e. informality, terseness, sincerity, lack of direction, and more. While none of these sound too catastrophic, they are often interrelated. As such, they tend to worsen any problem. For example, humor can cause informality, worsen ambiguity, and weaken sincerity. That being said, users should strive to avoid ALL these pitfalls.
3.) Dies ist die Mailbox von Herrn Mustermann. Bitte hinterlassen Sie eine Nachricht oder versuchen Sie uns unter dieser Nummer später nochmals anzurufen. Wir danken für Ihr Verständnis.

Voicemails can give an important message or inform the caller about the receiver's status. Thus, it is important for you to be clear while recording such messages. Here are a few examples of voicemail greetings to get an idea about appropriate messages that can be recorded the next time you leave your house or desk.
Recently one of my students asked a great question about voicemail in English and I thought that you might have the same question as well.