help! Windows Phone8.1 access authentication failed

I'm running FreeRADIUS Version 2.2 and Mysql for VPN access authentication on ubuntu 14.04, the Windows Phone8.1 clients send username as *Windows Phone\username*, the Mysql radcheck table doesn't include *Windows Phone\*, so access authentication failed. I 've googled this issue, and haven't found anything that worked. I have tried the methods in this post: http://lists.freeradius.org/pipermail/freeradius-users/2011-March/052650.htm..., but problem still exists.