Locate the response family
- 1xx ProvisionalRequest is progressing
- 2xx SuccessRequest succeeded
- 3xx RedirectionAnother destination
- 4xx Request failureInterpret this request
- 5xx Server failureServer could not fulfil
- 6xx Global failureFailure across destinations603 belongs here
Read the status with its request method and call leg. For example, 200 after REGISTER confirms a binding; it does not prove an INVITE succeeded.
What this response means
The called party or responsible system explicitly declines the session under global-failure semantics. It is broader than a single endpoint reporting busy here.
Read it in context
A user refusal or policy decision can generate a decline. Intermediaries may map business rules into this result. It does not by itself establish spam classification, incorrect credentials or an unreachable device.
Administrator checks
Identify the source and compare the destination’s current policy and user action. Check whether forwarding, do-not-disturb or screening explains the refusal. Preserve privacy and avoid repeated test calls to someone who is intentionally declining.
Next action and common mistake
Do not bypass the recipient’s refusal by forcing alternate identities or uncontrolled retries. For an owned test endpoint, correct an unintended policy and verify expected accept/decline behavior. Keep ordinary 486 busy handling separate from 603 semantics.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.