FreePBX accepting then rejecting calls with bad credentials

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 [[email protected]:5] Set(“SIP/”, “TIMEOUT(absolute)=15”) in new stack
– Channel will hangup at 2014-02-26 19:50:23.067 EST.
– Executing [[email protected]:6] Log(“SIP/”, "WARNING,“Rejecting unknown SIP connection from"”) in new stack
[2014-02-26 19:50:08] WARNING[32171][C-000000cb]: Ext. s:6 @ from-sip-external: “Rejecting unknown SIP connection from”

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).