In 1922 the Bell System implemented "panel" dial switching in New York City. The system had to handle a high volume of calls and a wide diversity of central offices throughout New York City. This would've been impossible with Step-by-Step which was limited to a 10x10 point rotary switch. Panel translated the decimal exchange to codes for hundreds of trunks as needed. Panel used common control to do this. That is, it stored the telephone number and translated it as needed for switching needs.
The control functions of panel were similar to modern computer CPU instructions and I/O bus commands. An ingenius sequence switch, loaded with numerous cams, controlled the functions. Different parts of the switchgear would be selected, the unit would wait for a ready signal, send out a command, then wait for a response and act accordingly. Everything was run by a pulse train generator exactly the same as a CPU clock today.
Signals between devices were self checking to detect errors. All gear had internal error detection as well as test functions.
Remember this was all 1922, long before diodes and solid state devices, indeed, tube electronics were brand new and still being developed. It was all done with electro-mechanical relays. All equipment had to be extremely durable and precise to handle a high load volume and last in service a long time.