Alien Front Online featured 3-on-3 battles between human and alien units. The different craft available to each group all basically handled like tanks. It wasn't a game based on fast action, so there was time to plan attacks with other team members using the microphone.
There are offline missions in the game, but they didn't really add much. The only real lasting impression it left me was that it was a game that 56k modem'ers could play that supported voice chat. Not real time "everyone can chat" type chat but any one player in the game could record a voice message at a time, that upon completion of entry could be played back to either teammates or everyone in the game.
Unfortunately, AFO came in Summer 2001 I think. The servers were crappy and with Sega phasing out the Dreamcast, I'd say they had no real reason to make sure they ran well. With multiple ISP's I'd get frequent disconnects (and many other people did, too).
Unfortunately the microphone is of no use offline (as one would imagine), although I forget whether you can test out the mic feature offline -- sort of in the same way that you can type in PSO while playing an offline game.
AFO really made me want Propeller Arena, because I thought it was going to be a much better game and more worth the $30. Stupid terrorists.