SIP
603 Decline
The callee's machine was contacted successfully but the user explicitly does not wish to or cannot participate. Unlike 600, this may be because the callee doesn't want to communicate, not necessarily because they're busy.
When You See This Code
When the callee explicitly declines the call. This is an active rejection, not a busy or unavailable state.
How to Fix
The callee has declined. Respect their decision. Consider alternative communication methods (email, messaging).