If someone calls your Another Number and the conditions busy, no answer and unavailable are met, then the call will go to voicemail.
Voicemails that you receive will be transcribed into text (if quality allows) and will appear in the 'Recent' menu of the Another Number app.
There are two options when it comes to voicemail.
1. Forward calls to voicemail in 'Do Not Disturb' mode - calls to Another Number will be sent directly to voicemail. You can set this via Switchboard -> Do Not Disturb.
2. Let the call ring before diverting to voicemail. You will see the incoming call, let this ring through to voicemail, if you decline the call there is a possibility that your carrier will pick up the call sending the call to your carriers voicemail system.