In the Numbers management menu -> Voicemail section please go to Voicemail block.
You can use voice mail greetings in 2 ways:
Use the standard greetings message. The standard greetings messages are set by default to US-English. You can listen to the standard message and if you are satisfied with this message, you don't have anything else to do.
Customize the greetings message: you can upload a pre-recorded message in .wav format.
Greetings messages are played when calls reach you voice mail.