Best solution is to buy a 3rd Party Fax/Modem (~$50)
You really shouldn't have to buy a 3rd-party modem.
One of the other links on Apple's Discussion Boards lists how to issue "Terminal" and "Finder" commands to have the Apple USB Modem work in 32-bit kernel-mode (which, as Larry from VA notes involves holding-down the "3" and "2" keys when booting).
But to set this up you need to be in 64-bit kernel-mode, apparently.
You should also use the EXACT version of the driver specified in the link (not a newer one, as I did (it didn't work)).
Note also that the link specifies the modificaiton dates of the drivers in European format (dd/mm/yy).
If you have a Time Machine backup that goes back far, you may not need to use the "Pacifist" app to extract the old files.
Under Lion, use "Applications":"Utilities":"System Information":"Software" to verify if you are booted into 64-bit mode -- "64-bit Kernels and Extensions" should say "yes"; otherwise you are booted into 32-bit mode -- "64-bit Kernels and Extensions" should say "no".
Go to page 3 of the 2nd link, which is:
apply the prescribed Finder steps found there, and then use Terminal to issue the following command:
sudo kextcache -v 1 -a i386 -a x86_64 -m /System/Library/Caches/com.apple.kext.caches/Startup/Extensions.mkext /System/Library/Extensions
The resulting obscure warning message on my machine under Lion 10.7.3 is:
JMicronATA.kext does not declare a kernel dependency; using com.apple.kernel.6.0.
Created mkext archive /System/Library/Caches/com.apple.kext.caches/Startup/Extensions.mkext.