I have a bunch of calls that look like this.
2011-11-18 00:27:10SIP/xx.xx...unknown"unknown" sANSWERED13
Is this someone fishing?
The short answer is yes. Ensure "Allow Anonymous Inbound calls" is set to NO in FreePBX General Settings. It should be set to this by default.
To prevent these connection attempts from showing up in the call logs go to:
http://yourserverip/admin
Tools>Advanced Administration>Asterisk SIP Settings>
Set "Allow SIP Guests" to "NO"
The reason we do not set it to that by default is because some SIP trunk providers are not compatible with that setting. If you have trouble with your SIP trunk provider after changing that setting then try change it back.