Hi James!
You might want to have your people look at http://community.freepbx.org/t/fpbx13-rc-g722-and-1-way-audio-even-to-voicemail/31042/74?u=marbled and http://forum.yealink.com/forum/showthread.php?tid=8330 ...
Yealink phones are amongst the phones getting a bad reputation because of the problem mentioned in both of those threads...
Have a nice day!
Nick