In Star Trek: Voyager, which episode shows when the Federation first found out that Voyager had survived?
The episode where Voyager makes initial contact with the Federation is Voy: Message in a Bottle. The EMH is transferred onto a Federation vessel and communicates Voyager's predicament to Starfleet.
This was subsequently followed up by a two-way channel of communication in Voy: Pathfinder, courtesy of Reg Barclay and the MIDAS communication array.
It may interest you to know that prior to this episode they'd already made contact with two different Alpha Quadrant powers; A Romulan in Voy: Eye of the Needle and two Ferengi in Voy: False Profits but unfortunately their situation wasn't communicated to Starfleet.