Earlier this year, I wrote about my problems sending faxes using Windows 8.1 Pro.
Apparently, there is fix for that now.
I haven't tested it yet. It was included in the November 2014 update rollup for Windows RT 8.1, Windows 8.1, and Windows Server 2012 R2, and I have the updated FXST30.DLL on my system. Hope it works ;-)