did you purchase this new !? if so there will prolly be some software on the installation cd/floppy disk that contains drivers to operate the modem as a hands free speaker phone
altho not very portable lol as yer wired to yer puter so spose a longer cable is needed lol !!
but on a serious note usually you get "supervoice" fax/voice software with most voice modems and you can also search google and download it its pretty easy to setup all you really need is a headphone/mic combo headset and away you go after the software is installed
its a not great solution but it works !
note this usually does not allow you to make voice calls on the internet using a net phone !