In this project, we demonstrate the 4 line telephone systems with full signaling and switching functions similar to those of the central office systems. Dial tone, busy tone, and ring tone are provided during call progress. Ringing is generated at the receiving end of the phone being called. Switching employs integrated circuit (IC) matrix switches on four buses.
http://instruct1.cit.cornell.edu/courses/ee476/FinalProjects/s2001/pr57/REPORT.htm