Terminals 1 & 6 were originaly designed as a means to additional power to anything plugged into any sockets.
The idea was a small plug into mains transformer. the DC cable has a PSTN plug at the end. with + & - voltage on 1 & 6
That way anything more than a basic phone could have additional power requirements fed by the existing telephone cabling.
As it turns out any equipment needing additional power are designed with its own mains plug or transformer. so the terminal 1 & 6 idea never took off. We still have that need to make sure our telephone sockets are within a couple of metres of a mains socket today for DECT, answer machines, alarm systems routers, faxes, telex machines etc etc.
Was a great idea, just that as the rest of the world didnt pick up on it, so many appliences were manufactured with the idea of a mains socket, or the designers just didn't know
