Asterisk behind NAT error

If asterisk behind NAT,how to configure client behind NAT?
I have set nat=yes in sip_extention.conf for the user. But the user still cant connect with my NAT?

are you forwarding the necessary ports on the server side?

Check that your router/fw are properly configured.
That ports 5060, 6061 and RTP ports (10000-20000) are not blocked and forwarded to your asterisk box.