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. You don't want to overdo it or people will get bored at best. This may have different implications for disabled users. The use in a contact center for quality monitoring of voice recordings is an excellent example.