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, receives 401 Unauthorized, and again sends the same REGISTER packet without the appropriate SIP Authentication header.

That means that MOR's IP address is blocked on 3CX for some reason.

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/