Skype/dingotel both support proxy. U don’t need to be worried about their config………..it is a easy task …….

DingoTel uses various technologies to perform VoIP. The first is Microsoft's Direct Play Voice which is also used extensively in gaming. This allows excellent support for dealing with firewalls and NAT. Iit is a p2p application.......

yes netmeeting can do voice conference……  ..but in low bandwidth it is not good enough. ......... I think Skype is better than netmeeting….but Dingotel, it is becoming quite famous for voice chatting and easy configuration...

hmm... there r many... but depends on how many ppl u r going to have conference with .. u can use yahoo .. or skype.. or........behind proxy u can try dingotel