iLBC is a good low bitrate codec, offering compression similar to G.729 however the MOS is higher when there are some lost packets.
ILBC mean Internet Low Biterate Codec
Dynamic Payload Numbers
iLBC utilizes dynamic payload number in the SIP SDP. FreeSWITCH supports dynamic payloads. However, some clients do not correctly. Look out for this when testing iLBC.
See also: Dynamic Payloads