I suspect this is an intentional blackhole built into the FreePBX dialplan to slow down and stop a type of attack. Here is an excerpt from the asterisk console:
Executing [s@from-sip-external:5] Set(“SIP/173.239.164.42-000000d4”, “TIMEOUT(absolute)=15”) in new stack
– Channel will hangup at 2014-02-26 19:50:23.067 EST.
– Executing [s@from-sip-external:6] Log(“SIP/173.239.164.42-000000d4”, "WARNING,“Rejecting unknown SIP connection from 173.208.194.242"”) in new stack
[2014-02-26 19:50:08] WARNING[32171][C-000000cb]: Ext. s:6 @ from-sip-external: “Rejecting unknown SIP connection from 173.208.194.242”
Can anyone tell me if there is a way to have FreePBX turn off this default context so calls without authentication are refused at the asterisk level (never enter the dialplan).