Great write up, background and tinkering- a homemade PBX system! Eight telephone extensions with roughly telco spec voltages and currents (48V onhook, 90VRMS 20Hz sinusoidal ringing, about 25mA loop current offhook.) Ring trip is sub-spec but can handle at least 3 “500” type rotary dial telephone sets in parallel without false tripping. Lines are not balanced, nor is one side ground. [via] Link.