Here is the page with the LTmodem drivers. Installing from source is easy. The driver you need depends on the kernel you're running. Many people have put alot of effort into developing this modem driver for linux and in contrast to the above posts it is a viable solution. As a motter of fact when you buy a computer preloaded with linux (Wal_Mart Linspire, TigerDirect Linspire, Dell preloaded with Linux) that is the modem that is used
LTmodem driver download
For the standard Slackware 2.4.31 kernel use ltmodem-8.31a.tar.gz. If you go with the 2.6.13 kernel there is a patched driver out there that you find by googling using "ltmodem" and "kernel 2.6" as search terms.
The modem runs as fast as any hardware modem since you are usually limited by line speed.
The modem also supports fax using efax-gtk.