Panasonic KX-TD500 System Reference Manual page 247

Conditions (1) Appearance of the call indication in case 2.
If more than one outside call (via CO lines of the same trunk group)
comes in under the following situation, the second call will appear on
the next GCO button of the same extension.
(Example)
Trunk Group 01
Incoming Mode (Day):
DIL 1: 1
1. GCO-01 is in use at Ext.100.
2. Indication of the call which comes in on Ext.100 via a CO line of
Trunk Group 01 appears on another GCO of Ext.100.
1.
GCO-01
GCO-01
GCO-01
Ext.100
3-D-29
CO Lines Direct Termination
CO Line 10101
CO Line 10102
CO Line 10103
CO Line 10104
2
Ext.100
Ext. 100
Ext. 101
GCO-01
GCO-01
GCO-01