Preemption Behavior on Low Priority Calls

A 180 ringing response is sent to the far end only when a call appearance is allocated for the incoming precedence call.

The following table illustrates the preemption behavior of the low priority call's status.

Table 1. Preemption Behavior on Low Priority Calls

Low Priority Call's Status for Preemption

Behavior

Connected

The call is terminated with a BYE request containing a preemption Reason header, and a local preemption tone is played for a configurable duration or until the user hangs up, whichever comes first.

Locally Held

The call may be terminated with a BYE request containing a preemption Reason header.

Alerting

A 486 Busy Here response is sent to the far end containing a preemption Reason header.

Dial Tone or Setup

When the final call appearance is in the dial tone or setup (digit collection) state (including consultation calls) and a precedence call arrives, no action is taken until the new outgoing call is of higher priority or is not is determined. If the call is of lower priority, then the call is not placed and a preemption tone is played for a configurable duration or until the user hangs up, whichever is less. If the call is of the same or higher priority, then the incoming call is terminated by sending a 486 Busy Here response to the far end containing a preemption Reason header.

Preceding

If the final call appearance is in the dial tone or setup (digit collection) state (including consultation calls) when a precedence call arrives, no action is taken until it can be determined whether the new outgoing call is of higher priority or not. If the call is determined to be of lower priority, then the call is not placed and a preemption tone should be played for a configurable duration or until the user hangs up, whichever is less. If the call is determined to be of the same or higher priority, then the incoming call is terminated by sending a 486 Busy Here response to the far end containing a preemption Reason header.