Panasonic KX-TD500 System Reference Manual page 1058

Parallel Connect
Message Waiting Indication
When pressing the NEXT key, this screen changes as follows:
Station Station
Equipment
Equipment
No. 1 (1/4)
No. 1 (2/4)
Pressing the PREV key changes the screen in reverse order.
Parallel Connection with SLT is available only PITS telephone interfaced
with HLC or DHLC card is selected.
If PITS telephone interfaced with PLC card is selected, "-" will appear in
Parallel Connect field and parallel connect assignment is not available.
The setting of "Lamp" is valid only when the extension is an SLT with
MESSAGE lamp which is interfaced with the KX-T96175 (MSLC) or KX-
TD50175 (ESLC).
If an extension card other than the KX-T96175 and KX-TD96174 is
installed, "—" appears and this item cannot be assigned.
Note:
Be sure to select "None" for this setting if an SLT without MESSAGE
lamp is interfaced with the KX-T96175 or KX-TD50175.
Station Station
Equipment Equipment
No. 1 (3/4) No. 1 (4/4)
10-G-6
Station
- - -
Equipment
No. 2 (1/4)
Station
Equipment
No. n (4/4)