Emergency/priority lines are used e.g. on UK trackside rail infrastructure and it is important that an incoming call from emergency circuit must always ring. If an emergency line rings on the phone, it must always ring with a specific ring type, i.e. be distinguishable from a non emergency call.

Description of the feature may be found also in Feature Description chapter 5.32.12. High priority lines in a keysystem

A single KCSU phone (primary device) has a number of SEC phones (PRIM signoff), a few of which are SEC lines of type 'emergency'.

For the primary device KCSU PRIM BUSYRING=ALERT

The SEC lines used for 'emergency' are marked HIGHPRIO:

DSSPRIO=HIGHPRIO is set in KCSU PRIM config of those 'emergency' SEC numbers (signoff)

SDAT DSSLINE is also set for the these lines.

The current functionality in the scenario with high priority lines is described below:

Primary device busy talking, new incoming call to high priority line starts ringing with high prio corresponding tone and keeps ringing until high prio call is answered or caller hangs up.

Primary device busy ringing, new incoming call to high priority line is signaled with corresponding high prio tone (ring tone is changed accordingly if there was not already a high prio call ringing on the device) and keeps ringing high prio until high prio call is answered or caller hangs up.

If phone is hooked off, device continues ringing with same high prio ringtone.

Even if another normal prio call is answered on the device, device will continue ringing with high prio tone.

Primary device busy talking, 2 new incoming calls (normal and high priority), device starts ringing with corresponding tone for high prio line as soon as the high prio call reaches the device and keeps ringing with this tone until high prio call is answered or caller hangs up.

Primary device busy ringing, 2 new incoming calls (normal and high priority), ring tone is changed to high prio as soon as the high prio call reaches the device, in case there was not already a high prio call ringing on the device, and keeps ringing with this tone until high prio call is answered or caller hangs up.

In case the normal priority call comes in after the high priority call, device keeps ringing with high prio melody.

Primary device ringing with incoming normal priority call, phone is hooked off and a high priority line key is pressed to make an outgoing call, incoming call is silenced because of outgoing high prio call.