Red5phone to PBX call problem

hello,

I can make a call between red5phone to webphone or other webphone and can hear any audio both ways but When I try to call red5phone to the external pbx I can hear it but the person who at the pbx cant hear me.I suspect that this is codec issue but do not know how to solve problem.the external pbx use g729 and g711 codec.I work at WinXP and use external sip server.

any help will be appreciated.