Digital: A Love Story is a Visual novel for Windows, Mac OS X and Linux created by Christine Love. Set "five minutes into the future of 1988" it tells the story the protagonist's attempts to uncover a mystery hidden on the Bulletin Board Systems (BBSs) of the era.
The player's interface with the game is rather unique, in that play takes place by logging in to replicas of 1980s Bulletin Board Systems then reading and replying to the posted messages. The messages that the players sends are never explicitly revealed, but their contents can be inferred from replies received from other characters in the game.
Logging in to the various BBSs presented in the game requires the player to manually enter telephone numbers and passwords.
Source: Wikipedia, "Digital: A Love Story", available under the CC-BY-SA License.