Voicemail messages that reside on the phone company's servers expire after a certain period of time (5 days for Read messages and 20 days for Unread messages). The expired messages are deleted automatically from the phone company's servers. Once a voicemail message is deleted, there is no way to recover it. You can, however, save messages on your phone by moving them to the Archive folder.
The maximum message limit for all folders combined (Inbox, Archive, and Trash) is 1,250. Once you reach this limit, you will get a message telling you to delete some messages before you can receive new messages. Moving messages to the Trash folder does not reduce the 1,250 count; however, deleting messages from the Archive folder does reduce the count.
The Archive folder may be on an SD card or in internal storage, depending on how the phone defines its main storage.
You may also archive messages by forwarding them to an email address. There is an Auto-Forward to email feature for Premium service subscribers that automatically forwards messages to a designated email address.
For further Visual Voicemail support, go to the customer care page on your service provider's web site.