SIP response codes – Difference between ‘603 Decline’ and ‘486 Busy Here’

(Above) A 6xx response type is known as a “doomsday” response code.

While lecturing on SIP response codes, I’ve had several students ask for an explanation of the case usage for a ‘603 Decline’ and a ‘486 Busy Here’.

The short answer is that all 6xx responses will terminate SIP dialog, and any pending searches. Alternatively, a 486 response would indicate that the user is not available at a particular request-URI.

I have constructed a video that demonstrates the difference between these two responses. It might even answer a few questions you didn’t even know you had.

In the following YouTube video, Examining the 486 ‘Busy Here’ and 603 ‘Decline’ SIP response codes, I have 3 phones connected to the same SIP proxy, Kamailio, and running a trace with Wireshark.

SIP Downloads *.pcap

This video mentions two accompanying (*.pcap) files. These can be found here: (posting soon)