backport fix for g729
Backport MRs:
asterisk-chan-voicemngr!30 (merged) : Set a default codec for incoming calls with a pre-negotiated result to avoid some unexpected failure transcoding that leads to the channel hangup.
!110 (merged) : Ignore transcoding failure to avoid unexpected disconnects for asterisk unsupported codec during the codec sync period.
Edited by Wenpeng Song