The Aqua Box was used to thwart the FBI's "Lock-in-Trace" system, that prevents a caller from ending a call, forcing the call to remain active even after hanging up, so that they can trace the caller's location. Lock-in-Trace works by cutting into a phone line through a three-way connection and generating the voltage to maintain the call connection. The Aqua Box simply lowers the voltage level on a phone line, preventing the Lock-in-Trace device from maintaining the necessary voltage to keep the line open, possibly even shorting out the Lock-in-Trace device itself.
The Aqua Box is now obsolete, as calls can now be traced instantly, requiring whole new techniques to be developed. Although, Lock-in-Trace remains a staple in film and TV shows to add drama and suspense as investigators trace a call, and as a means to showoff the technological savvy of a criminal or protagonist hacker.