Interplexing beacon

An interplexing beacon is a type of long range subspace communications transmitter used by the Borg to maintain contact with the Collective across long distances.

Upon assimilating a starship it is not unusual for the Borg to construct an interplexing beacon to establish contact with the collective. The Borg have done so on the USS Monitor in 2377 and USS Enterprise in 2063 when Borg from the 24th century attempted to establish contact with the collective of that era. (ST game: Away Team and TNG movie: First Contact)

In at least one alternate timeline the Borg also used interplexing beacons which could transmit through time, Starfleet located one such device in the wreckage of a Borg Cube in the Beta Quadrant, this particular beacon was stolen in 2390 by Chakotay and Harry Kim in an attempt to communicate with Seven of Nine in 2375 to prevent the destruction of the USS Voyager that had created that timeline. (Voyager episode: Timeless)
 * Given the recent point of divergence it doesn’t seem too improbable that temporal iterplexing beacons exist in the regular timeline as well.