[asterisk-users] Extensions busy/congested and "circuit-busy"

[Guy M Guyadeen]

Our pbx is a Fedora Core 4 box running Asterisk 1.2.6. It has a  
public IP and is publicly accessible (no NAT, firewall, etc.).
Our Sipura 841 phones are in several locations, all NAT'ed behind PIX  
firewalls.
The system used to work flawlessly, but now any extension we dial  
comes back as "busy". On the Asterisk CLI, I always see something like:

Got SIP response 488 "Not Acceptable Here" back from {our IP address}
     -- SIP/{extension}-d544 is circuit-busy
   == Everyone is busy/congested at this time (1:0/1/0)


Would extensions.conf or CLI debug output be helpful?

Thank you in advance!
Guy