Of Course! You have complete control over your inbox, and all voicemails are stored in your smrtPhone account waiting for you to review.
When a voicemail is received in the "Inbox", they will show up in the list and you can listen to the recording and process it further.
All you need to do in order to customize your voicemail message and very simply add your own recording is to head over to Setup ---> Voicemail.
Simply click the 'Edit' button and select how you’d like to set your message.
Note: The voicemail message will be the default voicemail for anyone who calls you.