In 2374 Voyager used an ancient Hirogen relay network to send the Doctor to a Starfleet ship, letting Starfleet know Voyager had survived.
In 2374 Seven of Nine found a vast communications network of relay stations built long ago by the Hirogen, one of which reached the edge of the Alpha Quadrant. Voyager used it to transmit the Doctor's program to the experimental USS Prometheus.
The Doctor arrived to find the Prometheus taken over by Romulans. Working with the ship's own Emergency Medical Hologram Mark II, he helped retake it. He then reported to Starfleet, which had declared Voyager lost, and returned with word that Starfleet would try to help bring the crew home.