Visual Voicemail is a system that will e-mail your voicemails, in .wav format, to an e-mail account of your choice. Once delivered to your in-box you may see who left the message and listen to it if you so choose.
If you have not set up Visual Voicemail or had done so previously, had it work for a while, and then had it stop working, take a look at the attached instructions, follow them and sign up for Visual Voicemail from scratch. Do this even if you had done so before the system stopped working for you.