439
SIP
439 First Hop Lacks Outbound Support
The first outbound proxy does not support the SIP outbound mechanism (RFC 5626) that the UA is trying to use.
On this page
When You See This Code
When a UA configured for SIP outbound connects to a proxy that doesn't support the outbound option-tag.
How to Fix
Upgrade the outbound proxy to support RFC 5626, or configure the UA to not require outbound support.
Specification
Compare With
Guides
401 Unauthorized vs 403 Forbidden: When to Use Each
Security & Authentication
SIP Response Codes: Understanding VoIP Call Flow
Real-Time Protocols
SIP Call Flow: Understanding Session Setup
Real-Time Protocols
WebRTC Signaling with SIP and WebSocket
Real-Time Protocols
RFC 3261: SIP Protocol Deep Dive
Protocol Deep Dives
TLS 1.3: Faster, Simpler, More Secure
Protocol Deep Dives
Glossary
Status Code
Request Method (HTTP Verb)
TLS Handshake
Authentication
TCP/IP
Port Number
NAT (Network Address Translation)
Socket
Request-Response Model
RFC (Request for Comments)
Connection-Oriented
Full-Duplex
SRV Record
SIP Dialog
Signaling
SIP Registrar
STUN (Session Traversal Utilities for NAT)
TURN (Traversal Using Relays around NAT)
SDP (Session Description Protocol)
SIP INVITE
SIP Proxy Server
SIP Redirect Server
SIP Transaction
SIP User Agent
SIP Trunk
RTP (Real-time Transport Protocol)
SRTP (Secure Real-time Transport Protocol)
SIP ACK
SIP BYE
SIP CANCEL
SIP OPTIONS
SIP SUBSCRIBE/NOTIFY
B2BUA (Back-to-Back User Agent)
SBC (Session Border Controller)
Codec Negotiation
SIP Response Classes
VoIP (Voice over IP)
Presence (SIP)
PBX (Private Branch Exchange)
MFA (Multi-Factor Authentication)
TLS Versions
Frequently Asked Questions
What is SIP 439 First Hop Lacks Outbound Support?
The first outbound proxy does not support the SIP outbound mechanism (RFC 5626) that the UA is trying to use.
When do you see SIP 439 First Hop Lacks Outbound Support?
When a UA configured for SIP outbound connects to a proxy that doesn't support the outbound option-tag.
How to fix SIP 439 First Hop Lacks Outbound Support?
Upgrade the outbound proxy to support RFC 5626, or configure the UA to not require outbound support.