439 4xx Client error RFC 5626
SIP 439 First Hop Lacks Outbound Support
The registrar supports SIP Outbound, but the first-hop proxy does not.
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Common causes
- Mixed SIP Outbound support across the path.
How to fix it
- Disable SIP Outbound on the UA, or upgrade the edge proxy.
Seeing 439 on calls through a router or firewall? Check that SIP ALG is disabled. It causes a surprising number of SIP errors. Read why.
About 4xx responses
The request has a problem or cannot be completed at this server, usually because of something on the requesting side or the target user.