[VoiceOps] SIP 404 Not Found vs 603 Decline

[Alex Balashov]

Our experience has been that it is difficult to predict this, due to the way in which 6xx cause codes have been misused throughout the supply chain.

As RFC 3261 states, 603 Declined is for those cases in which the UAS has authoritative knowledge that the destination cannot be reached in principle, including by any other means. Proxies are not supposed to serially fork beyond a gateway returning this class of error, and UAs aren't supposed to attempt a new call leg to that destination either.

Practically, ‎603 Declined is a popular catch-all epithet for "cannot complete the call", much as 503 Service Unavailable, and route-advancing upon receipt of such is, accordingly, more the norm than not. This means that it is subject to as much unpredictable adaptation to ISUP / ISDN as anything else. 

So, it's impossible to say exactly what the PSTN caller's experience will be with any certainty.

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Alex Balashov | Principal | Evariste Systems LLC
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