Apple has released North American and International English versions of the Apple Modem updater.
The updater supports the internal modem supplied with PowerBook G3s, iMacs, Blue-&-White Power Macintosh G3s and Power Mac G4s (PCI and accelerated graphics-port variants).
It contains new firmware for the modem, an updated command set and an improved country selector. Apple says that a slight speed-drop may be experienced after installation, but claims this is offset by data-transfer improvements, thanks to "better modem throughput-speed". Apple claims the updater also makes internal modems more stable.
The update is available from Apple's Software Downloads page.