Telsius Prime was a Delta Quadrant mining colony where con artists posing as Captain Janeway and Tuvok cheated the miners out of a cargo of bolomite in 2376.
The colony appears in the sixth-season episode Live Fast and Prosper. Dala and Mobar, wearing imitation Starfleet uniforms, tell the miner Orek a moving story about orphans in need and promise goods in exchange for his bolomite, then leave with the ore and deliver nothing.
Orek tracks down the real Voyager and attacks it, which is how Janeway learns that her identity is being used. The fraudsters had copied the ship's database after tricking Neelix and Paris.
Janeway captures Dala, lets her escape with a hidden tracker and has the Doctor impersonate her to recover the stolen property. The loot is handed back to the victims, Orek among them.