Plural Exchange Network Or Interconnection Patents (Class 379/219)
- Signalling path distinct from trunk (e.g., CCIS) (Class 379/230)
- Central office-to-PBX signalling (Class 379/231)
- PBX to central office signalling (e.g., direct outward dialing) (Class 379/234)
- Voice frequency signalling over trunk (Class 379/235)
- D.C. signalling over trunk (Class 379/236)
- Interexchange trunk circuit (Class 379/240)