Hi!
I wonder if the problem is in any way related to G722, quite a few people here have been having problems with G722 and, amongst other thing, Yealink phones, see http://community.freepbx.org/t/fpbx13-rc-g722-and-1-way-audio-even-to-voicemail/31042/74?u=marbled
Good luck and have a nice day!
Nick