asterisk users - Sep 2004 - no sound from jsphone, the voicemail ok

Hi, i just installed * following the basic instructions found in onlamp.com,
all seems ok, i can make calls from one windows 2k to another using jsphone,
the sip phone register ok with *, but when one phone ansers the call, i can
hear nothing, if i dont answer the call, the voice mail wav file plays ok,
any hint?