Outtrigger was a somewhat unique release for Sega when it hit arcades back in 1999; the company was hardly well-known for its FPS escapades, and the coin-op's combination of joystick and trackball presented some barriers for casual arcade players more attuned to simple, accessible experiences.
Nonetheless, the game became something of a cult classic when it was ported to the Dreamcast in 2001, complete with support for the console's mouse-and-keyboard control setup and—most importantly of all—online play (a feature that was sadly removed for the game's European release).
Playing against other people was a big draw in Outtrigger, so when Sega turned off its servers shortly after the new millennium was welcomed in, it reduced its appeal somewhat. Thankfully, in recent years, we've seen many Dreamcast games get renewed online support thanks to the efforts of the community, and it seems that Outtrigger is up next.
As reported by Dreamcast Live, developers flyinghead and @Shuouma_DC are currently working on reintroducing online play in the game via unofficial servers. You'll also need to get your Dreamcast back online to benefit from this, something our handy guide can help with.