SIP
604 Does Not Exist Anywhere
The server has authoritative information that the user indicated in the Request-URI does not exist anywhere. Unlike 404, this is a definitive, global statement.
When You See This Code
When the user definitively does not exist on any server in the network. The server has authoritative knowledge, not just local.
How to Fix
The user does not exist. Verify the SIP URI or phone number. Unlike 404, retrying on other servers will not help.