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 an Archive folder. The Archive folder has a limit of up to 500 messages or the available free space, whichever limit is reached first. If you reach the limit for the Archive folder, you'll see a message prompting you to delete some messages to make space.
The Archive folder may be on an SD card or in internal storage, depending on how the phone defines its main storage. Phones that define their main storage as internal storage will store the archived messages to the local path directory "/mnt/sdcard/smvvm/".
For further Visual Voicemail support, go to the customer care page on your service provider's web site.