I'd like to shed some light on this mechanic, which is intentionally kept hidden during the game. Every character has both a bad ending and a romance ending. Getting the bad ending is fairly simple, you just need to give that character an answer that make him feel more depressed for three times during the hotline phase.
Reach the romance ending is a bit more tricky. After giving the correct answers over the phone, the character will eventually show up at the bar and ask you a question. If you answer all three of those events correctly, you'll unlock the chance to go on a date with him/her!
There's a bit of rng involved in when characters appear, but every ending can be achieved with some patience. I hope that clears things up. And thanks for playing our game! ;)