3CX registration fails

Registration from 3CX to MOR fails with a similar error:

Registration at TEL_0322211033 has failed. Destination (sip:11.22.33.44:5060;transport=TCP;lr) is not reachable, DNS error resolving FQDN, or service is not available.

PCAP shows that 3CX sends a REGISTER packet, MOR sends 401 Unauthorized back to 3CX, and 3CX sends the same REGISTER packet to MOR without the appropriate SIP Authentication header.

That means that MOR's IP address is blocked on 3CX for some reason, 3CX does not receive 401 Unauthorized and tries to register again from the start.

The solution to enter MOR's IP address into the Whitelist on 3CX as described here: https://www.3cx.com/docs/allow-deny-ip-addresses

See also

• https://www.3cx.com/docs/sip-trunk-registration-authentication/