I am looking for the canonical definition of the “Allow Anonymous Inbound SIP Calls” option under “Asterisk SIP Settings” in FreePBX. Also, how does it relate to "Allow SIP Guests"?
This option is to allow calls not associated with any of your trunks. Usually you want that disabled. To make it more clear, if this were a VoIP phone with this option on, the device would ring at random times since it would accept any "INVITE" mainly coming from sip scanners.