First, gamers require a third-party USB modem. Two are confirmed by From as acceptable so far: Sun Electronics' OnlineStation and I-O Data Device's P2GATE. Other modems are in the works, though, and may prove to work if their manufacturers make it so.
Once they're connected to a phone line, players have to directly dial the number of the opponent they want to duel -- From will provide no online matching servers. Once one player dials and the other answers, the sending player sets the options for the game (timelimit, map, and AC) and the fight begins.
The sending party picks up the phone bill (there's no charge to play, since no matching server is involved), and all the other problems of a phone connection apply -- call waiting and other types of interference can break up the connection. This is certainly not the best possible online gaming solution, but it is something. From can, if nothing else, always claim they were first.
Screenshots from Impress Game Watch.