Visual voicemail has access to a visual interface, which shows the list of playable voicemail messages and sometimes can include decryption of those messages.
Because Google Voice provides such a competitive and feature-rich service that not only includes free VoIP and SIP calls across North America and internationally at fantastic rates, in addition to a huge range of services such as voicemail, conference calls and voice transcription, it was inevitable that since its inception it would attract the ire of other competitors who would block it, or sue in courts to have the government legislate against it.