Using Multiple Subscriber Numbers (MSN) is the best way to direct incoming calls to the desired device or software component.

The ISDN line needs to be allocated multiple subscriber numbers by the network provider, and each device filters on a specific number.

Incoming calls to any of the MSNs arrive at the same ISDN line, but the SETUP will have a different "called party number" depending on the number being called. The devices will each be configured to respond to a different number.