Apple posted a Tech Info Library article (“PowerBook G3 Series, iBook: Voice Mail/Speakerphone Compatibility“) which says:
The modems in PowerBook G3 Series, PowerBook G3 Series (Bronze keyboard), and iBook computers do not support voice and cannot function as a speakerphone. The modem may work with some third-party voice mail applications, but Apple does not support the use of these applications.
That’s pretty much the whole of the article.
