that is fully hardware based. Most modems sold these days rely upon software in the computer to do the DSP (digital signal processing) work that converts the squeaks and squawks of the signal coming over the phone line into 100% Genuine Binary Goodness that your computer can actually use, and then reverses the process to squirt bits at the other computer. Generally, modem manufacturers don't like to write software drivers for Linux (because we're godless commies don'ya'know).
Even though the F/OSS movement has made a lot of progress on this ground in the past decade, your best bet is still a external hardware modem. It will cost slightly more (maybe $30 instead of $10) but you will be all around.
These are the three modems I usually recommend to people. One $30, one $40, the other $70.
http://www.newegg.com/Product/Product.asp?Item=N82E16825138108http://www.newegg.com/Product/Product.asp?Item=N82E16825104140http://www.newegg.com/Product/Product.asp?Item=N82E16825104135Here are similar choices at BestBuy, one $50 the other $60.
http://www.bestbuy.com/site/olspage.jsp?skuId=6422604&type=product&id=1077626890126http://www.bestbuy.com/site/olspage.jsp?skuId=4327785&type=product&id=1051384345922BUT! Why BestBuy? Have you considered a local or near-by Mom'N'Pop computer store? Usually they have solid prices that are the same or not much higher than the big block stores and you're dealing with someone who actually knows what they're talking about. There may even be a local Linux users' group they could hook you up with.