Cranial transceiver

From Memory Alpha, the free Star Trek reference

The cranial transceiver is an interplexing beacon implanted within the braincase of every Borg drone. Each transceiver contains a unique translink frequency that allows the drone to stay in contact with the collective. (VOY: "Timeless")

In an alternate version of the year 2390, where USS Voyager crashed after a quantum slipstream flight, the Doctor was able to extract Seven of Nine's cranial transceiver and determine its translink frequency. This allowed Harry Kim and Chakotay to contact Seven in the past and avert Voyager's destruction. Seven's translink frequency was 108.44236000. (VOY: "Timeless")