Lock and Trace. This system was used by the FBI to 'lock' into the location of the telephone user. An explanation.
The FBI can tap into a call, like this:
|/---- FBI computer
When they tapped in, they can blow some electricity trough the line.
All phone connections stay open by a certain level of electricity. When you hang up, the level of electricity decreases and the connection is terminated.
Lock and Trace keeps the electricity on the same level, even when you hang up. The phone rings immediately then.
The level of electricity keeps the same untill the tracing is over and the FBI knows who the phreak was.
The Aqua Box is connected to the phone line and needs energy to put itselfs on, the electricity level decreases and the FBI-computer stops tracing.
With ESS is Lock and Trace not necesary anymore - and the Aqua Box with it - and it isn't used anymore.