It is highly dependent on what type of modem you have as to whether it will work in Linux or not. Linux, including Fedora, will detect and install basic drivers for most any full hardware modem
that is installed. However, most computers that come with dialup modems, or the cheap ones you buy that fit into a pci slot are not of that type, but what are usually referred to as winmodems
modems, though there is a technical difference between those two types.
Fedora will ignore any such winmodem. There are a few winmodem types that have linux drivers available and can me made to work under linux. Try this:
lspci -v | grep Modem
Be sure to use a Capital M in the word Modem. I have two dialup modems installed on my box, one a conexant chipset winmodem and another a USR/3Com V.90 full hardware modem. The above command on my machine reports both devices, but Linux completely ignores the winmodem. The hardware modem is detected on bootup and works fine without having to install any drivers for it, just a bit of configuring in network-manager sets it up easily.
[paulm@speedie ~]$ lspci -v | grep Modem
05:06.0 Serial controller: 3Com Corp, Modem Division 56K FaxModem Model 5610 (rev 01) (prog-if 02 )
Subsystem: 3Com Corp, Modem Division USR 56k Internal FAX Modem (Model 5610)
05:08.0 Communication controller: Conexant HSF 56k HSFi Modem (rev 01)
So, first step, find out which type of modem you have and check back in here.