The 2600Hz supervision with MF in-band signaling on many American trunks resulted in the infamous "blue box" which enabled free calls to be made.
The device wouldn't work in Britain due to the different signaling systems, but the "black box" would certainly work on the old GPO system here.
As none of this is applicable to the modern system, it won't hurt to reveal the details.
The black box enabled free calls to be made, the only catch being that the device had to be located at the receiving end of the call. The trick involved was to use a fairly high value of resistance across the line, low enough to trip the ringing on the final selectors (i.e. cut off the ringing voltage to the line), but not low enough to complete the DC loop to signal an off-hook condition. It was the the latter which sent a supervision signal back to the originating exchange to indicate "call answered" and to start charging.
Having tripped the ringing, it was then possible to connect an audio path via isolating capacitors to the line and talk both ways. In some cases, the connection would time-out and release after a minute or two, but of course somebody could just call back again -- Hey, the calls were free!
The practice was, of course, illegal.
