Annoying Registration error

I am using Asterisk 1.8.9 server. Someimes, my asterisk CLI console is filled in with messages like this:
[Dec 30 22:19:00] NOTICE[30918]: chan_sip.c:24767 handle_request_register: Registration from ‘“645” sip:645@MY-ASTERISK-SERVER’ failed for ‘ARBITARY_IP:5404’ - No matching peer found

Different times, the number 646 and port number are different such as 643 and 5293 etc. I don’t have any explicit registration in my sip.conf, the arbitary IP is not my provider’s host either.
When this happens, whole console is filled in with these messages, hard to view or debug anything else. But it does not interfere with calls.

Any idea, where these messages are coming and how to stop them?
Thanks in advance.

A criminal.

Configure your firewall to block SIP from address ranges that should not be sending to you.

Consider changing the SIP port number, so they have to find that before they can start the main attack.

Use fail2ban to dynamically modify the firewall to block addresses that have previously failed (won’t help if the addresses differ every time).

Does any forum member has first hand experience with hosting vendors who can provide reliable asterisk hosting on physical server with all security issue taken care of and round the clock support.

If all security issues are taken care of it will be irrelevant as to whether or not it is Asterisk, it will just be a black box centrex VoIP server.