PJSip 'Allow Guests' - what does this do exactly?

Got 100 or so Avaya 1100 series SIP phones pointing to an old Asterisk box. Want to point them to a new asterisk box and use PJSIP. Got the test batch of phones working, but in the log over and over from all the phones i keep seeing:

res_pjsip/pjsip_distributor.c:347 log_unidentified_request: Request from ‘sip:company.net’ failed for ‘’ (callid: ff38ace5edbaab39) - No matching endpoint found

Now all the phones work using PJsip, no problems…just this above entry repeats every 30-60 seconds for all the sets.
If i set “Allow Guest” = YES under PJSIP in freePBX Advance SIP settings, the message goes away.

Just wondering what the system stops looking for when i activate this feature? I hope it would help me figure out what setting i’m missing.

EDIT: These model of phones ping the system every 60 seconds for a keep alive. Disabled the feature (sip_ping = no) and cleared up the repeating entry.