A beginner asking for help

The asterisk is behind NAT. I first set up two extension numbers and they worked fine. Then I tried to configure the trunk and the outbound routing. It shows at the Asterisk Info. that SIP has registered, but I just can’t call out. On the LCD screen it always shows 404. What does 404 mean?
The sip server I use can only call PSTN number but cannot be called in by PSTN. How should I set the Outgoing & incoming Settings for the trunk on this condition?

I find this in the log:

Feb 28 16:55:51 WARNING[2921] chan_sip.c: Forbidden - wrong password on authentication for INVITE to ‘“200” ;tag=as5e6b7599’

what password is this??

404 means not Found…

If you dont mind can you please paste your trunk configuration here?

Seems your outbound routing not propper, check your dial plan also…