!!!ANORAK ALERT!!!

Pavanne, it was the "pay-on-answer" principle that thwarted free calling, though these boxes were necessary to enable self-dialling of trunk calls as well as local calls (STD). As you say, it continued to be the case that each digit was signalled to the exchange by breaking the circuit the same number of times as the digit value. This rotated a mechanical selector the corresponding number of notches, thereby routing the call to the appropriate point on the next bank of selectors, and so on through successive banks, until all digits had been dialled.

These "step-by-step" eachanges, dependent on "loop disconnect" signalling were the brainchild of an undertaker called Strowger who lived in or around Kansas I think (Bill D?). He was driven to his invention because the manual switchboard in his town was run by his rival's wife, through which his rival was always the first to hear of local deaths. Strowger exchanges stayed at the heart of telecoms switching in the developed world from the 1920s right into the 1980s. (Many developing countries bypassed Strowger, going straight from manual to digital switching.) On some exchanges it was possible to lock a selector by tapping 11, and this in theory could threaten to put such an exchange out of action.

LTS, the reason 999 was preferred over 111 (which was as easy to find on the dial and much quicker to call) was a concern that calls might be generated by accidental line breaks (bumping the handrest, birds on phone wires etc). In the UK we somehow overlooked the 911 compromise.

Not mentioned on the site Shambles pointed to, but someone might be interested: ADVance was used for part of the east end because of a perceived social stigma attached to the original proposal, BEThnal Green. Options were limited as they were by then stuck with the 238 numerical code.