Thanks for your replies. I don’t fully understand some of the terminology (endpoints, AOR, … ), but let me try and make things clearer for you so you can help make things clearer for me 
What I mean by “place in the UI” is “this error is probably something that I configured incorrectly either in the FreePBX UI, or in the HT813 configuration UI”.
But while it should be straight-forward to go back in the screens and find out, I am actually having trouble finding it.
Here are some more details (I obfuscated my phone number which starts with 21 by overwriting 212212212 manually):
<--- Received SIP request (554 bytes) from UDP:10.0.0.3:5062 --->
REGISTER sip:10.0.0.10 SIP/2.0
Via: SIP/2.0/UDP 10.0.0.3:5062;branch=z9hG4bK1989114344;rport
From: <sip:[email protected]>;tag=1868922450
To: <sip:[email protected]>
Call-ID: [email protected]
CSeq: 2030 REGISTER
Contact: <sip:[email protected]:5062>;reg-id=2;+sip.instance="<urn:uuid:00000000-0000-1000-8000-C074AD90A2AE>"
Max-Forwards: 70
User-Agent: Grandstream HT813 1.0.13.3
Supported: outbound, path, gruu
Expires: 3600
Allow: INVITE, ACK, OPTIONS, CANCEL, BYE, SUBSCRIBE, NOTIFY, INFO, REFER, UPDATE
Content-Length: 0
<--- Transmitting SIP response (321 bytes) to UDP:10.0.0.3:5062 --->
SIP/2.0 404 Not Found
Via: SIP/2.0/UDP 10.0.0.3:5062;rport=5062;received=10.0.0.3;branch=z9hG4bK1989114344
Call-ID: [email protected]
From: <sip:[email protected]>;tag=1868922450
To: <sip:[email protected]>;tag=z9hG4bK1989114344
CSeq: 2030 REGISTER
Server: FPBX-16.0.26(18.13.0)
Content-Length: 0
[2022-12-21 15:02:39] WARNING[40600]: res_pjsip_registrar.c:1166 find_registrar_aor: AOR '' not found for endpoint 'LinhaMeo' (10.0.0.3:5062)
Some configuration screens, I don’t know which are relevant. Note that 10.0.0.10 is the FreePBX.
From what I gather from your replies, it seems…
- The error is caused by “initiative” of the HT813, trying to register with FreePBX, but FreePBX is not happy and rejects it
- I am probably quite confused about what entity (trunk? extension? user?) I should be configuring for this on the FreePBX. I think I only have a trunk. Should I create some other entity so that FreePBX is happy with the registration request?
BTW, I have the calls working in both directions, which is curious…
I also see this old thread, I don’t know if it is related: