Huge wrote:

Actually, the A/B boxes used "gongs" in the coin feed mechanism, so
that the operator could hear the coins running in. You're confusing
them with the later mechanical boxes, where inserting the coin wound
up a clockwork mechanism which as it unwound put 5K ohm loops across
the line, which could be automatically counted by a piece of equipment
in the exchange.

Cooo.... fascinating! Is there a decent web site out there which explains
the mechanisms involved in the old callboxes?