All external modems connected via a RS-232 serial port should work. This includes external ISDN adapters, although some of the extended features of external ISDN adapaters (such as multilink) may or may not work.
Internal modems are another story, however. There are many so-called "winmodems" available now. In fact, it seems that most PCI modems are winmodems. Some of them do have drivers for Linux now, but many of the drivers are often binary-only. (See the note on binary-only drivers.) See Linmodems.org for more information on Linux-supported winmodems.
Note that there are external USB winmodems on the market now, so be very careful when shopping for external modems.
Furthermore, many flash upgradable modems only have flash programs for Win95/NT. These modems cannot be upgraded under Linux.
A small number of modems come with DOS software that downloads the control program at runtime. These can normally be used by loading the program under DOS and doing a warm boot. Such modems are probably best avoided as you won't be able to use them with non PC hardware in the future.
Most 16-bit PCMCIA modems should work with the PCMCIA drivers. CardBus modems are usually winmodems much like PCI modems. Your best bet for now is to find a card that lists compatibility with DOS and Windows 3.1.
All that said, if a modem is known to have a real UART (or hardware UART emulation), whether it is ISA, PCMCIA, etc., it should work under Linux.
Fax modems need appropriated fax software to operate. Also be sure that the fax part of the modem supports Class 2 or Class 2.0. It seems to be generally true for any fax software on unix that support for Class 1.0 is not available.
An exception to this is the Linux efax program which supports both Class 1 and Class 2 fax modems. In some cases there can be a few (minor) technical problems with Class 1 modems. If you have a choice it is recommend to get a Class 2 modem.
See Appendix E Linux Incompatible Hardware for specific cards known not to work with Linux.
The following are other good resources for finding Linux-compatible modems:
Rob Clark's "Winmodems are not modems" page
Andrew Comech's "PCI modems and Linux" page
Andrew Comech's "Cheap /Linux/ Box" section on modems
Other useful documents include the following:
Below is a very incomplete list of modems currently known to work under Linux.
|3Com||3CP5613 Internet Gaming Modem||PCI|
The following is old information and may not be entirely correct. It may be removed in a future revision of this document.
Digicom Connection 96+/14.4+ - DSP code downloading program here
Motorola ModemSURFR internal 56K. Add a couple of line to RC.SERIAL to account for IRQ and ports if they are non-standard.
ZyXEL U-1496 series - ZyXEL 1.4, modem/fax/voice control program here
ZyXEL Elite 2864 series - modem/fax/voice control program here
ZyXEL Omni TA 128 - modem/fax/voice control program here
Also multimodem cards are supported by Linux.
Moreton Bay RAStel multimodem card Check here for Linux drivers.
The following modem is mentioned not to be supported
Aztech MDP3858 56.6 (PCI)