379 / | HD | TELEPHONIC COMMUNICATIONS |
219 | DF | PLURAL EXCHANGE NETWORK OR INTERCONNECTION {6} |
229 | DF | .~ Interexchange signalling {6} |
236 | DF | .~.~ D.C. signalling over trunk {1} |
237 | DF | .~.~.~ Pulse or digital signalling {2} |
239 | ![]() | .~.~.~.~ Using register-sender |