Make sure that your audio card is full duplex, check also for NAT and Firewalls

• http://gnomemeeting.org can reach also windows users

"C:\Program Files\Netmeeting\conf.exe"
file:///C:/Program Files/Netmeeting/conf.exe

PORT     STATE SERVICE   VERSION
1503/tcp open  imtc-mcs?
1720/tcp open  teleconf  Microsoft NetMeeting Remote Desktop Service