All working but Voice conversation not working

I am new to astersik. i have successfully installed asterisk on suse 11

with following pakage:

        Zoiper (softphone)

I am able to register softphone from asterisk cleints and able to call
and hold but voice conversation is not working at all.
Please tell me what would be the possible problem.
if have trun of suse firewall on asterisk server but its still not working, is there any other ? if there is please specify its location.
Please tell me what would be the problem that i m unable to talk, thanks.