The SBS 2011 Fax printer 32-bit drivers currently shipped with SBS 2011 are not compatible with Windows XP. They only work with Vista and newer. Attempting to send a fax on an XP machine with these settings does nothing and no error messages are displayed.
To solve, you can use the driver from SBS 2003 SP2, as these are compatible with the SBS 2011 Fax system. They can be found inside the Windows 2003 SP2 download (see below for detailed instructions), or directly from an SBS 2003 (located in c:\windows\system32\spool\drivers\w32x86\3. The files you need are these:
Most people won't have an SBS 2003 with SP2 anymore. They can still download the SP2 installer, and manually extract the necessary files. After downloading, run the installer with the /X option and specify a temporary location to extract all files. Then locate the CAB-file fxsclnt (in the i386 subdirectory). Extract the following files from the cab:
Rename these files by removing the FXS_ prefix so the names are as in the first list of files in this article (e.g. FXS_FXSAPI.DLL becomes FXSAPI.DLL).
Copy these files manually onto the XP clients:
Note that when removing the fax printer, you may have to do this procedure again. It's best to keep the driver files somewhere central so it's easy to deploy the driver onto future clients. You could also deploy the driver using a group policy.