A couple of years ago I converted a telephone handset into a lineman's handset to troubleshoot the PBX system at work. As a kid I'd always marveled at how two-w

A couple of years ago I converted a telephone handset into a lineman's handset to troubleshoot the PBX system at work. As a kid I'd always marveled at how two-way voice (and data!) communication and numbers could be transmitted over four wires, and it really blew my mind when I learned that just two are being used per line. We had pulse-tone rotary phones which were simple enough to understand - just count the pulses. I'm still working on completely grasping the combined tones used in touch-tone dialing, specifically the way they would have been interpreted at the switchboard before the introduction of digital PBX systems.