This is really about customer preference. Calling customers desire a choice. Some use self-serve, others never will. For a business to provide high levels of customer satisfaction, both are required. Business rules must always be worked into any solution.
Audio information needs to be presented quickly and to the point.
Text based information can go into much more detail, and generally does. The content of emails and voice mails is a good contrast, for example. Emails tend to be much more detail-oriented. I think listening to too much text can get your typical caller impatient and frustrated. People can process much more information more quickly visually than audibly.