A phase coil resonator was a type of engineering tool.
In 2370, Lieutenant B'Elanna Torres used one while trying to construct a prototype Automated Personnel Unit. (VOY: "Prototype")
The prop was also seen in the movie Men in Black (curiously as a universal translator) but was first seen on Voyager. It was later re-used as a Srivani medical device in "Scientific Method" and a Hirogen engineering tool in "Flesh and Blood".