Singing a song, using a fake "hello," and pretending that your voice actually belongs to the voicemail are some amusing options for cell phone greetings. Remember that some greetings may be inappropriate for professional environments.
Using a song or rhyme is a great way to create a funny greeting, for example, a small poem that begins with "Roses are red, violets are blue."
A fake "hello" works by pretending to answer the telephone, leaving a gap for the other person to start talking, then saying, "I'm sorry, I'm not available right now..."
A funny greeting may reference famous movies or TV shows, with quotes altered to refer to an answering machine.
Bear in mind that anyone who calls frequently may end up becoming annoyed by a novelty greeting.