iOS App:
To listen to your voicemails on your smrtPhone Mobile App, start in the Inbox tab from the lower menu on the app. It defaults to your text message inbox, and if you select the menu icon (≡) from the upper left of this screen it will give you an option to view your voicemails.
Individual Voicemails
Voicemails assigned to any Groups you are in
Once you see the Voicemail you'd like to listen to you can:
Play
Delete
Mark Unread
Mark Read