S0-Module 420
The S0 module 420 has two ISDN basic connections; one external port and one internal/external switchable. Up to two ST 25 or ST 30 digital system phones can be connected to the internal ISDN line.





S0-Module 440
The S0 module 440 has four switchable ISDN connections. This module can easily operate of up to eight ST 25 or ST 30 digital system phones.