You can record a custom voicemail greeting right from the app (iOS only). 

  1. Tap the Settings menu in the top-left of the upper toolbar.
  2. Tap Voicemail Greeting.
  3. Select Custom, then tap Record
  4. Make your recording, then tap Stop. 
  5. Tap Play to preview, or Record to try again. 
  6. Once you’re happy with your greeting, tap Save in the upper toolbar. 

It is not yet possible to customize your voicemail greeting from the Android app.