Here is some information about modems that will work on Linux. I recently had quite a time with attempting to configure some older isa modems for one of my machines. I finally gave up on the old ones and got new models. I tried an external modem that hooks to the serial port. It worked without much problem. Here are the main hardware details:

Machine: 	Dell Optiplex GXa 266 mhz processor Pentium II 64 megs of ram.

Extermal Modem: U.S. Robotics 56K Faxmodem.
	 	on the bottom of the unit on a white tag is the following info:
	 	Modem Model number 5686 
 	 	Fc 0701
	 	the smaller tag has these numbers:
	 	2ABLYBDD1812
	 	Product # 005686-05
	 	The copyright date stamped on the bottom of the case is 2002.

For the external modem I disabled the two onboard serial ports of the GXa. I used RedHat 8 version 2.4.18-14 . The internet connection wizard found the modem; and from there everything went smoothly.

I also tried the following internal modem on the same machine with the same RedHat system; and I might add that on the outside of the box under "Minimum System Requirements", Linux ( kernel 2.3 ) was included in the list:

Internal Modem: U.S. Robotics Model 0778.
On the circuit board of the modem was the following:
SN 3MCCY1KF1707
Product
64-005610-02
USR-1.012.0776-F

the chips are apparently 3com:
3com 1.016.1777-00
TL16CFM700PGE
98C 49AJ28W

and

1.016.1223
US ROBOTICS
D17807PJ
HHU 4CA5DEW

For this modem all I did was install it in the machine; the rest was done by the RedHat 8 system. It was the easiest modem installation I've ever done. It worked flawlessly. I left the onboard serial ports on "Auto". The internal modem is slightly faster than the external one, but they both work just fine.