You can record a custom greeting for your voicemail or use the default greeting for Google Voice. Record up to 10 voicemail greetings and select the one that you want callers to hear. Record a new greeting. When you use the Google Voice app, you can record a greeting up to 3 minutes long.
A word of warning: These greetings will not do you any favors if you're in the midst of a job hunt or work in a conservative industry. Always remember your target personas. If there's a chance they won't appreciate your sense of humor, opt for a straightforward greeting instead.

Hello, it’s obvious you have bad timing, because nobody is home. Please leave your name, telephone number, and a brief message in a voice similar to mine, and your call will be returned as soon as humanly possible.
With this method, you don’t need a third-party app. Call the person you want to chat with, then tap the “Add Call” button on your phone’s screen. Dial your own phone number—the one you’re calling from. This will take you to your voicemail. Once your voicemail has started recording, tap the “Merge Calls” button on your screen. Now, you’ll record your phone conversation onto a voicemail message. When you’re done with the call, you can tap the share button to export it to your Mac via Airdrop, to the Voice Memos app, or transfer it via email or text message. The file is recorded as an M4A, so depending on what you want to do with it, you may need to convert it to MP3 before uploading it to other apps.