It seems Grandstream's issue. It might be a phone's bug. As you can see, the phone doesn't put Authorization: or Proxy-Authorization: header in REGISTER. It kept sending REGISTER with the same "CSeq: 2149 REGISTER" even though the SIP Server replied "401 Unauthorized" response. Let you reboot the ...

Who returned an error response such as 481 and 407?

Did you find them in the Session Logs page ? Or Error logs page?

If you find them in the Session Logs, the callee returned the error response.
If you find them in the Error Logs page, the Brekeke SIP Server returned the error response.