Welcome to the community!
Do you have the iPhone Visual Voicemail add-on? New plans have the regular voicemail (up to 3 messages, each 3 minutes in length that can be saved up to 3 days). The regular voicemail will send you a SMS when you receive a voicemail. I believe if you wish to use the Visual Voicemail functionality of the iPhone, you will need to get the add-on. I'm not sure whether the iPhone Visual Voicemail is available as standalone add-on or only available with the value pack.
Hope this helps 😀