Christopher McDonald provided the voice of Superman. In The Last Son of Krypton: Part I (1996), he provided the voice for Superman's father, Jor-El.
Though the Justice League existed since Bruce's time as Batman, this is their debut in the DC Animated Universe.
On the special feature, Bruce Timm said that Tim Daly was available but the producers chose to use McDonald's voice as a sign of Superman's advanced age. Ironically, Daly was later unavailable to voice Superman in Justice League and was replaced by George Newbern.
While talking to Terry McGinnis on the phone, Bruce Wayne is shown in the Batcave's garage working on the Batmobile's second version seen from Batman: The Animated Series.
This episode indicates that the Fortress of Solitude's location is a secret kept even from the other JLU members but Batman (Terry McGinnis) reveals that his predecessor knows its location. The Justice League Unlimited episode "For the Man Who Has Everything" confirms this fact when Batman (Bruce Wayne) and Wonder Woman visit the location to celebrate Superman's birthday.