asterisk users - Sep 2005 - IAX2 how to disable VAD ?

Hi All ;)

I use VoipBuster with a IAX2 trunk between them and my 1.09  Asterisk.
I use the G729 codec over my poor 64ko isdn line but  when i putt a  
call on hold, le personn i call does't hear the music on hold (or  
just for some seconds)
It seems that the VAD is enable on the IAX link. How can i disable it ?
Is there something to add into my iax.conf file ?

Thanks for the reply ;)
Julien.