Although users rarely see it, the technology behind the telephone switch is actually quite interesting. In the first hundred years or so of their existence, they evolved from manually operated distribution boards to computer-controlled systems, but between these two stages, dialing and switching were carried out in an electromechanical manner. This is because of the invention of step switch, which is a pulse operated relay, which can connect an incoming line to one of multiple outgoing lines.
Public telephone switches contain hundreds of such switches, but as [dearuserhron] shows, smaller systems can be built with fewer components: the cadr-o station is built around a stepping switch. Although it looks quite complex, the only other components are a pile of ordinary 24 V relays and some power supplies. Together, they form the smallest telephone exchange, which can connect up to ten mobile phones.
However, it does not have all the functions of a larger system, because there is only one voice circuit, and all mobile phones will be automatically connected to the circuit. Nevertheless, it allows users to dial a number and then make another call ring, which may be enough for a family or its current hacker space. It also shows how to apply relatively simple technology to build an extremely complex system.
[dearuserhron] wrote an in-depth article on the working principle of electromechanical telephone exchange, which may be helpful to anyone who wants to design such a system for their home. For a more comprehensive introduction to analog telephony, check out our analysis of rotary phones in the 1970s.