What happens when you lift the handset?

A customers line is controlled by its Line Circuit. This is two relays and a uniselector. The uniselector is a semi circular arc of contacts (typically 24) and a set of wipers which rotate round the arc to make contact with any of the 24 outlets.

The wipers are metal arms carrying the incoming line. When someone lifts the handset the line circuit relays detect this and operate the uniselector. The uniselector hunts around its 24 outlets looking for one that is not in use. Each outlet leads to a 2 motion selector.

A line needs to be connected to a 2 motion selector before dialling can commence as the 2 motion selector is the basic building block of the exchange which processes the dialled digits.

Although lines could be wired directly to their own 2 motion selector this would be inefficient as most lines are idle most of the time. 2 motion selectors are complex and costly pieces of kit so there are only enough to handle the anticipated number of simultaneous calls. It's the job of the subscribers line circuit & uniselector to find a free 2 motion selector when a call is made.