Prior to that they offered a third party Apple-badged comparatively low-tech acoustic coupler.
As of September 2009 it is no longer available in the US Apple Store, but it still works (at least for fax) as of Mac OS X version 10.6.2.
A device driver for the modem was introduced with Mac OS X version 10.4.3.
Apart from using the Apple USB Modem for Internet dial-up and faxing, it is also being suggested as a low cost line interface (aka FXO interface) for telephony applications, such as for telephone systems (software PBX) and answering machine software.
The highly miniaturized product, about the size of a cigarette lighter and with a 4.6-inch long USB cable, won a RED DOT design award for good design.