A SIP trunk binding doesn't have to be down(Not responding to keep alive options) for surecall to kick in, a PBX just needs to send one of the below SIP codes and we will unconditionally forward the call to the supplied surecall number.

The following list, are SIP responses codes that will trigger an unavailable call forward (which means Surecall on a PBX with DIDs that have it enabled):

Please note that it's the code that is important and not the definition in (). The ones listed are the standard definitions to give an idea of what the SIP response code represents.

• 404 (Not Found)
• 408 (Request Timeout)
• 410 (Gone)
• 413 (Request Entity Too Large)
• 414 (Request-URI Too Long)
• 415 (Unsupported Media Type)
• 416 (Unsupported URI Scheme)
• 417 (Unknown Resource-Priority)
• 420 (Bad Extension)
• 421 (Extension Required)
• 423 (Interval Too Brief)
• 493 (Undecipherable)
• 500 (Server Internal Error)
• 502 (Bad Gateway)
• 503 (Service Unavailable)
• 504 (Server Time-out)
• 505 (Version Not Supported)
• 513 (Message Too Large)
• 603 (Decline)