To access Visual Voicemail, you must have already Set Up Voicemail. Alternatively, you can check your voicemail messages by selecting the Phone app from the home screen > select and hold the 1 key > if prompted, enter your voicemail password > follow the prompts to listen to your messages. If you have forgotten your existing voicemail password, you will not be able to access voicemail until you reset your voicemail password.
When a new voicemail is received, the  Voicemail icon will appear in the Notification bar.