It's an Americanism . . . Their telephone "jacks" can be wired with one pair (circuit) on the innermost two pins and a second pair (circuit) on the outermost two pins. I'm sure
Ezzer will be able to provide the finer details . . .
If it helps take a look at an RJ11 (plug) and the connections thereof. As it has six pins, the potential of an inner pair, a middle pair and and outer pair should then become apparent.
