What response is sent to a new caller if the called party on a PSTN device does not want to switch calls?

When a caller is trying to connect to a called party on a Public Switched Telephone Network (PSTN) device, and the called party does not wish to switch to the new call, the appropriate response is a "603 - Decline." This response indicates that the called party has received the request but has chosen to reject it for some reason, effectively informing the caller that their attempt to establish a connection has been declined.

In the context of SIP (Session Initiation Protocol), the "603 - Decline" response is specifically used to tell the caller that the invitation to establish a session is not acceptable. The caller is informed that the attempt to connect is rejected, allowing them to take appropriate action based on this response.

Other response codes serve different purposes: for example, "200 - OK" indicates that a call or media session has been successfully accepted, "486 - Busy Here" signifies that the called party is currently busy and unable to answer, and "404 - Not Found" indicates that the called party is unreachable or does not exist in the network. While these responses convey different statuses, they do not reflect the called party's conscious choice to not engage in the call, which is precisely what the "603 - Decline" response