spyke is somewhat correct. Most of the modems out there not have driver support on UNIX/Linux/BSD because they are Winmodems - that is, most of the modems’ work is done inside the Windows operating system, which helped to bring down the price of the hardware and get more people online faster. UNIX/Linux/BSD crowd aren’t terribly pleased with the prospect of trying to write all that code again, and especially not doubly again to use in Asterisk, because there are better options available.
There are many modems that wouldn’t meet the quality necessary to handle voice communications, and that is on top of the driver issue, above.
But, there are modems that will work with the Zaptel drivers, even though they are not from Digium. Check out the following Wiki page for details:
Many of these clones can be purchased in the $10 to $15 range, though the quality might be a little less than you would expect.