I've been busting my head for a few days trying to figure out how to make telepathy-rakia to see all gstreamer codecs. My particular interest is G.729. Do you have any clue whether some config file has to be modified? farstream seems to have it enabled but Wireshark says it's not offered by the phone:

cat /usr/share/farstream/0.1/fsrtpconference/default-codec-preferences
################
# Audio codecs #
################

[audio/SPEEX:8000]
clock-rate=8000

[audio/SPEEX:16000]
clock-rate=16000

[audio/AMR]

[audio/G729]

[audio/ILBC]

Codec seems to be installed:

gst-inspect-0.10 | grep 729
rtp: rtpg729depay: RTP G.729 depayloader
rtp: rtpg729pay: RTP G.729 payloader