First you define when you want calls to go to voicemail. You can divert all calls, only when you're busy, when you don't answer the phone, etc. Then, depending on the conditions you set, some calls will get diverted to voicemail.

 

As an example, let's say you've set Voicemail to divert when unanswered. A friend calls and you're away from the phone. The call go to voicemail and they hear a friendly automated voice requesting them to leave a voice message. You return to your phone and see an SMS saying that you've got a new voicemail. You call 771 to check the message.