These issues have been resolved with a modem driver update called PB 3400 Modem Driver Update. This driver should resolve these issues and prevent unnecessary hardware replacements. The PB 3400 Modem Driver Update is available on Apple's Software Updates sites.
Important Notes: - The PowerBook G3 does not need this modem driver update because the software that ships with this computer already has the update installed.
- These drivers are also included with Mac OS 8.1 update so customers that upgrade to 8.1 will not need the unsupported drivers.
This article can help you locate the PB 3400 software update mentioned here: