SIP 603 Decline

What SIP 603 means

The call was actively declined rather than failing. Someone or something chose to reject it, which on outbound business calls often means a carrier policy refusal or a blocking rule rather than a person pressing reject.

Global failure (6xx)
Sent by The called party, or a carrier acting on policy
Common

What SIP 603 Decline actually means

The callee was successfully contacted but explicitly declined the call, and the decline applies to the user rather than to one device. That global scope is what separates it from a 486: a 486 says this phone is busy, a 603 says do not try elsewhere either.

What causes SIP 603 on a business phone system

The person pressed rejectThe straightforward case, and not a fault at all.
A call blocking or screening rule rejected itAnonymous call rejection, a blocklist, or a screening service will decline without ringing anyone. Common when calling from a withheld or unrecognised number.
The carrier declined on policy groundsPresenting a caller ID the carrier will not accept, or calling a barred destination, can be refused with a decline. This is the version that matters to businesses, because it looks like the recipient rejected you when in fact the call never reached them.
A fraud system intervenedUnusual patterns, particularly to expensive international destinations, trigger automatic refusals. Worth recognising rather than fighting, since it is protecting you from a large bill.

How to fix SIP 603

Check whether it is one destination or manyOne number declining is likely their blocking rule. Many declining points at your caller ID or a barring rule on your account.
Verify the caller ID you presentConfirm it is a number assigned to you and correctly formatted. An unpresentable CLI is a leading reason for calls being declined by carriers and screening services.
Check your account barring and fraud limitsWhere a class of destination is being declined, it may be your own protection rather than the far end.
Consider how your number appears to recipientsA business whose calls are being screened out has a reputation problem worth addressing, not just a technical one.

Fixing the underlying problem

This page explains the code. These guides walk through the fix in detail.

Questions about SIP 603

What does SIP 603 Decline mean?

The call was refused deliberately. That can be the person pressing reject, but on business outbound calls it is just as often a carrier or screening service declining on policy, for example because of the caller ID you are presenting.

What is the difference between SIP 603 and SIP 486?

A 486 means that device is busy and the call could still be answered elsewhere. A 603 is a decision on behalf of the person that applies everywhere, so there is no point trying another destination for them.

Related SIP response codes

Source. SIP 603 Decline is defined in RFC 3261 section 21.6.2 and registered in the IANA SIP Parameters — Response Codes. The causes and fixes above are drawn from our own experience supporting UK business phone systems.

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