1. Try to update to the latest available version.
service webcallserver update
2. Try to change codec priority in flashphoner.properties
Example: H.264 is in priority h264,vp8
codecs=opus,alaw,ulaw,g729,speex16,g722,mpeg4-generic,telephone-event,h264,vp8,flv,mpv
Example: VP8 is in priority...