600 6xx Global failure RFC 3261
SIP 600 Busy Everywhere
The callee is busy and no other endpoint will accept the call.
Typical Q.850 / ISDN cause: 17 (User busy)
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Common causes
- All of the user's devices are busy.
How to fix it
- Retry later. Treat it like 486, but don't try alternate destinations.
Seeing 600 on calls through a router or firewall? Check that SIP ALG is disabled. It causes a surprising number of SIP errors. Read why.
About 6xx responses
The request cannot succeed anywhere. The caller should not retry it elsewhere.