Hi Zafar,
I’ve used the H.323 Softphone from EyeP Media in the past behind firewalls/NAT etc. with success.
Their web site is: http://www.eyepmedia.com
I think they were moving to a SIP based product but you should still be able to get hold of the H.323 version – it came with a thirty day trial so that you could test it before purchasing.
Hope this helps.