There is a known issue where some users have reported not being able to hear voicemails. It relates to the sound settings on your phone and the way Burner was built.
Burner was designed to play voicemails through your phone's speaker or a headset. When your phone is set to silent, the speaker on your phone is disabled. Our app cannot override the sound settings on your phone, so if your phone is set to silent when you try to listen to a voicemail, you will be unable to hear the message.
We understand that this can be confusing and we are working on an alternative way to play voicemails for users.