Panasonic KX-NCP500 Features Manual page 273

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If the last digit of the MSN is "0", all terminal devices on the same ISDN port receive the call simultaneously,
if the specification of each terminal device is available. To use the MSN whose last digit is "0" as an
individual MSN, system programming is required for each ISDN port.
PC Programming Manual References
3.33 [1-1] Slot—Port Property - BRI Port—ISDN Extension
3.35 [1-1] Slot—Port Property - PRI Port—Extension Setting
Feature Guide References
1.1.1.4 Multiple Subscriber Number (MSN) Ringing Service
1.21.1 Integrated Services Digital Network (ISDN)
Feature Guide 273