Here's a reference.
http://www.zoom.com/products/dial_up_external_serial.html I have an old zoom modem which I think is the model before this in the Spares box; not needed much since we went broadband, but it did do all the voicemail stuff, and faxes. It worked reliably and effectively for several years and I recommend them.
I am not sure if they are available for USB, but there is something quite reassuring about old-style serial technology for this kind of application, because I *know* I can make it work even in an emergency with even an emergency boot diskette and some sort of old DOS program like Telix or ProComm -- no need to worry about drivers and the like.
