The OpenSIPS returns codes and their scenarios are listed:
SIP 400 Bad Request; Missing OSP elements or bad message.
SIP 400 Bad Request - Calling number invalid; Calling number is invalid.
SIP 400 Bad Request - no route header in BYE message; SIP BYE message does not have Route header.
SIP 401 Unauthorized; OSP token is invalid, invalid wire device, invalid transport or source device.
SIP 401 Not Phone Number; Not used.
SIP 403 Forbidden - Call is blocked; Route blocked.
SIP 404 Route Not Found - No supported route; No supported route (may have unsupported destination, such as H323 device).
SIP 404 Route Not Found; Route not found.
SIP 405 Method Not Allowed; Source may not originate.
SIP 480 Temporarily Unavailable - Server not ready; OSPrey server not start, OSPrey server timeout or OSPrey server not ready.
SIP 483 Too Many Hops; Too many hops.
SIP 484 Address Incomplete; Invalid called number.
SIP 488 Unknown Domain; Not used.
SIP 500 Internal Server Error; Internal error such as memory error.
SIP 500 Unsupported Message; Unsupported SIP Message.
SIP 503 Service Not Available; Routing feature off, license exceeded, inbound device throttle or Unspecified server error (all other errors).
SIP 503 Service Not Available - Call cannot be completed; Call cannot be completed by any destination.
SIP 513 Message Too Big; SIP message is too big.
SIP 603 Decline; Fraud detected.
For NexOSS-FC, the following codes are not used
SIP 400 Bad Request - no route header in BYE message
SIP 403 Forbidden - Call is blocked
SIP 404 Route Not Found - No supported route
SIP 503 Service Not Available - Call cannot be completed
Response code is configurable. For example, some customers want 503 instead of 404 for Route Not Found.