Codec problem

My IAX phone use codec g711 can call out through PRI channel but use codec g729 can not call out. Why?

Error prompted in asterisk as follow:

May 26 18:21:10 WARNING[797]: channel.c:1891 ast_request: No translator path exists for channel type Zap (native 68) to 256
May 26 18:21:10 NOTICE[797]: app_dial.c:764 dial_exec: Unable to create channel of type ‘Zap’

who can help me