Panasonic KX-TDA100D Features Manual page 224

1.21.1 Integrated Services Digital Network (ISDN)
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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.
Feature Guide References
1.1.1.4 Multiple Subscriber Number (MSN) Ringing Service
224 Feature Guide