An ISDN tie line connecting system connects an ISDN tie line between two private branch exchanges via an ISDN office line using an ISDN primary rate interface. The ISDN tie line connecting system includes a tie line interface, a provisional interface for connecting an office line interface, and an interface...http://www.google.de/patents/US5331629?utm_source=gb-gplus-sharePatent US5331629 - System for connecting ISDN tie line via ISDN office line
